diff --git a/src/doomdata.h b/src/doomdata.h
index d066277a7..6b3002cdb 100644
--- a/src/doomdata.h
+++ b/src/doomdata.h
@@ -29,6 +29,7 @@
 
 // Some global defines, that configure the game.
 #include "doomdef.h"
+#include "m_swap.h"
 
 //
 // Map level types.
@@ -220,6 +221,13 @@ struct mapsubsector_t
 	WORD	firstseg;	// index of first one, segs are stored sequentially
 };
 
+#pragma pack(1)
+struct mapsubsector4_t
+{
+	WORD	numsegs;
+	DWORD	firstseg;	// index of first one, segs are stored sequentially
+};
+#pragma pack()
 
 // LineSeg, generated by splitting LineDefs
 // using partition lines selected by BSP builder.
@@ -231,6 +239,22 @@ struct mapseg_t
 	WORD	linedef;
 	SWORD	side;
 	SWORD	offset;
+
+	int V1() { return LittleShort(v1); }
+	int V2() { return LittleShort(v2); }
+};
+
+struct mapseg4_t 
+{
+	SDWORD v1;
+	SDWORD v2;
+	SWORD angle;
+	WORD linedef;
+	SWORD side;
+	SWORD offset;
+
+	int V1() { return LittleLong(v1); }
+	int V2() { return LittleLong(v2); }
 };
 
 
@@ -238,18 +262,40 @@ struct mapseg_t
 // BSP node structure.
 
 // Indicate a leaf.
-#define NF_SUBSECTOR	0x8000
 
 struct mapnode_t
 {
+	enum
+	{
+		NF_SUBSECTOR = 0x8000,
+		NF_LUMPOFFSET = 0
+	};
 	SWORD 	x,y,dx,dy;	// partition line
 	SWORD 	bbox[2][4];	// bounding box for each child
 	// If NF_SUBSECTOR is or'ed in, it's a subsector,
 	// else it's a node of another subtree.
 	WORD 	children[2];
+
+	DWORD Child(int num) { return LittleShort(children[num]); }
 };
 
 
+struct mapnode4_t
+{
+	enum
+	{
+		NF_SUBSECTOR = 0x80000000,
+		NF_LUMPOFFSET = 8
+	};
+	SWORD 	x,y,dx,dy;	// partition line
+	SWORD 	bbox[2][4];	// bounding box for each child
+	// If NF_SUBSECTOR is or'ed in, it's a subsector,
+	// else it's a node of another subtree.
+	DWORD 	children[2];
+
+	DWORD Child(int num) { return LittleLong(children[num]); }
+};
+
 
 
 // Thing definition, position, orientation and type,
diff --git a/src/p_setup.cpp b/src/p_setup.cpp
index d9b285c03..5a6cc671f 100644
--- a/src/p_setup.cpp
+++ b/src/p_setup.cpp
@@ -1029,6 +1029,22 @@ static void P_LoadZNodes (FileReader &dalump, DWORD id)
 }
 
 
+//===========================================================================
+//
+// P_CheckV4Nodes
+// http://www.sbsoftware.com/files/DeePBSPV4specs.txt
+//
+//===========================================================================
+
+static bool P_CheckV4Nodes(MapData *map)
+{
+	char header[8];
+
+	map->Read(ML_NODES, header, 8);
+	return !memcmp(header, "xNd4\0\0\0\0", 8);
+}
+
+
 //===========================================================================
 //
 // P_LoadSegs
@@ -1037,6 +1053,7 @@ static void P_LoadZNodes (FileReader &dalump, DWORD id)
 //
 //===========================================================================
 
+template<class segtype>
 void P_LoadSegs (MapData * map)
 {
 	int  i;
@@ -1053,7 +1070,7 @@ void P_LoadSegs (MapData * map)
 
 	memset (vertchanged,0,numvertexes); // phares 10/4/98
 
-	numsegs = lumplen / sizeof(mapseg_t);
+	numsegs = lumplen / sizeof(segtype);
 
 	if (numsegs == 0)
 	{
@@ -1085,14 +1102,14 @@ void P_LoadSegs (MapData * map)
 	{
 		for (i = 0; i < numsegs; i++)
 		{
-			seg_t *li = segs+i;
-			mapseg_t *ml = (mapseg_t *) data + i;
+			seg_t *li = segs + i;
+			segtype *ml = ((segtype *) data) + i;
 
 			int side, linedef;
 			line_t *ldef;
 
-			vnum1 = LittleShort(ml->v1);
-			vnum2 = LittleShort(ml->v2);
+			vnum1 = ml->V1();
+			vnum2 = ml->V2();
 
 			if (vnum1 >= numvertexes || vnum2 >= numvertexes)
 			{
@@ -1222,12 +1239,13 @@ void P_LoadSegs (MapData * map)
 //
 //===========================================================================
 
+template<class subsectortype, class segtype>
 void P_LoadSubsectors (MapData * map)
 {
 	int i;
-	DWORD maxseg = map->Size(ML_SEGS) / sizeof(mapseg_t);
+	DWORD maxseg = map->Size(ML_SEGS) / sizeof(segtype);
 
-	numsubsectors = map->MapLumps[ML_SSECTORS].Size / sizeof(mapsubsector_t);
+	numsubsectors = map->MapLumps[ML_SSECTORS].Size / sizeof(subsectortype);
 
 	if (numsubsectors == 0 || maxseg == 0 )
 	{
@@ -1244,11 +1262,11 @@ void P_LoadSubsectors (MapData * map)
 	
 	for (i = 0; i < numsubsectors; i++)
 	{
-		WORD numsegs, firstseg;
+		subsectortype subd;
 
-		(*map->file) >> numsegs >> firstseg;
+		(*map->file) >> subd.numsegs >> subd.firstseg;
 
-		if (numsegs == 0)
+		if (subd.numsegs == 0)
 		{
 			Printf ("Subsector %i is empty.\n", i);
 			delete[] subsectors;
@@ -1257,8 +1275,8 @@ void P_LoadSubsectors (MapData * map)
 			return;
 		}
 
-		subsectors[i].numlines = numsegs;
-		subsectors[i].firstline = firstseg;
+		subsectors[i].numlines = subd.numsegs;
+		subsectors[i].firstline = subd.firstseg;
 
 		if (subsectors[i].firstline >= maxseg)
 		{
@@ -1390,6 +1408,7 @@ void P_LoadSectors (MapData * map)
 //
 //===========================================================================
 
+template<class nodetype, class subsectortype>
 void P_LoadNodes (MapData * map)
 {
 	FMemLump	data;
@@ -1397,13 +1416,13 @@ void P_LoadNodes (MapData * map)
 	int 		j;
 	int 		k;
 	char		*mnp;
-	mapnode_t	*mn;
+	nodetype	*mn;
 	node_t* 	no;
 	WORD*		used;
 	int			lumplen = map->Size(ML_NODES);
-	int			maxss = map->Size(ML_SSECTORS) / sizeof(mapsubsector_t);
+	int			maxss = map->Size(ML_SSECTORS) / sizeof(subsectortype);
 
-	numnodes = lumplen / sizeof(mapnode_t);
+	numnodes = (lumplen - nodetype::NF_LUMPOFFSET) / sizeof(nodetype);
 
 	if ((numnodes == 0 && maxss != 1) || maxss == 0)
 	{
@@ -1416,8 +1435,8 @@ void P_LoadNodes (MapData * map)
 	memset (used, 0, sizeof(WORD)*numnodes);
 
 	mnp = new char[lumplen];
-	mn = (mapnode_t*)mnp;
-	map->Read(ML_NODES, mn);
+	mn = (nodetype*)(mnp + nodetype::NF_LUMPOFFSET);
+	map->Read(ML_NODES, mnp);
 	no = nodes;
 	
 	for (i = 0; i < numnodes; i++, no++, mn++)
@@ -1428,10 +1447,10 @@ void P_LoadNodes (MapData * map)
 		no->dy = LittleShort(mn->dy)<<FRACBITS;
 		for (j = 0; j < 2; j++)
 		{
-			WORD child = LittleShort(mn->children[j]);
-			if (child & NF_SUBSECTOR)
+			int child = mn->Child(j);
+			if (child & nodetype::NF_SUBSECTOR)
 			{
-				child &= ~NF_SUBSECTOR;
+				child &= ~nodetype::NF_SUBSECTOR;
 				if (child >= maxss)
 				{
 					Printf ("BSP node %d references invalid subsector %d.\n"
@@ -3590,17 +3609,34 @@ void P_SetupLevel (char *lumpname, int position)
 			// This just means that the map has no nodes and the engine is supposed to build them.
 			if (map->Size(ML_SEGS) != 0 || map->Size(ML_SSECTORS) != 0 || map->Size(ML_NODES) != 0)
 			{
-				times[7].Clock();
-				P_LoadSubsectors (map);
-				times[7].Unclock();
+				if (!P_CheckV4Nodes(map))
+				{
+					times[7].Clock();
+					P_LoadSubsectors<mapsubsector_t, mapseg_t> (map);
+					times[7].Unclock();
 
-				times[8].Clock();
-				if (!ForceNodeBuild) P_LoadNodes (map);
-				times[8].Unclock();
+					times[8].Clock();
+					if (!ForceNodeBuild) P_LoadNodes<mapnode_t, mapsubsector_t> (map);
+					times[8].Unclock();
 
-				times[9].Clock();
-				if (!ForceNodeBuild) P_LoadSegs (map);
-				times[9].Unclock();
+					times[9].Clock();
+					if (!ForceNodeBuild) P_LoadSegs<mapseg_t> (map);
+					times[9].Unclock();
+				}
+				else
+				{
+					times[7].Clock();
+					P_LoadSubsectors<mapsubsector4_t, mapseg4_t> (map);
+					times[7].Unclock();
+
+					times[8].Clock();
+					if (!ForceNodeBuild) P_LoadNodes<mapnode4_t, mapsubsector4_t> (map);
+					times[8].Unclock();
+
+					times[9].Clock();
+					if (!ForceNodeBuild) P_LoadSegs<mapseg4_t> (map);
+					times[9].Unclock();
+				}
 			}
 			else ForceNodeBuild = true;
 		}
diff --git a/src/p_writemap.cpp b/src/p_writemap.cpp
index db758631f..7a052cb3b 100644
--- a/src/p_writemap.cpp
+++ b/src/p_writemap.cpp
@@ -172,8 +172,10 @@ static int WriteVERTEXES (FILE *file)
 	return numvertexes * sizeof(mv);
 }
 
+
 static int WriteSEGS (FILE *file)
 {
+#if 0
 	mapseg_t ms;
 
 	ms.offset = 0;		// unused by ZDoom, so just leave it 0
@@ -190,10 +192,14 @@ static int WriteSEGS (FILE *file)
 		}
 	}
 	return numsegs * sizeof(ms);
+#else
+	return 0;
+#endif
 }
 
 static int WriteSSECTORS (FILE *file)
 {
+#if 0
 	mapsubsector_t mss;
 
 	for (int i = 0; i < numsubsectors; ++i)
@@ -203,10 +209,14 @@ static int WriteSSECTORS (FILE *file)
 		fwrite (&mss, sizeof(mss), 1, file);
 	}
 	return numsubsectors * sizeof(mss);
+#else
+	return 0;
+#endif
 }
 
 static int WriteNODES (FILE *file)
 {
+#if 0
 	mapnode_t mn;
 
 	for (int i = 0; i < numnodes; ++i)
@@ -224,7 +234,7 @@ static int WriteNODES (FILE *file)
 			WORD child;
 			if ((size_t)nodes[i].children[j] & 1)
 			{
-				child = NF_SUBSECTOR | WORD((subsector_t *)((BYTE *)nodes[i].children[j] - 1) - subsectors);
+				child = mapnode_t::NF_SUBSECTOR | WORD((subsector_t *)((BYTE *)nodes[i].children[j] - 1) - subsectors);
 			}
 			else
 			{
@@ -235,6 +245,9 @@ static int WriteNODES (FILE *file)
 		fwrite (&mn, sizeof(mn), 1, file);
 	}
 	return numnodes * sizeof(mn);
+#else
+	return 0;
+#endif
 }
 
 static int WriteSECTORS (FILE *file)