I uhhh, wouldn't attempt to just try and copy-paste a full level into another (only mm3, 4, and 5 are compatible with each other)
But what you would do is just only copy-paste each segment seperately, not all at once.
Like do the "TSA table" block data first, then the Structure table, then the screen data.
Assuming you were to copy like Dust Man mm4 ...to Needle Man mm3
let's actually try that and see what happens ?? The level data format is waaaay down in the document.
just do a find-search for "Level Data" in notepad until you get to where there's a bunch of ========= below it.
Dustman's level data starts at 4A010 hex - Needleman starts at 00A10 hex - Have 2 fceux's open to do this
16x16 block data (400 bytes): copy 4A010-4A40F in megaman 4 ROM --- to 1B10 in megaman 3 ROM
pal/block type (100 bytes): copy 4A410-4A50F in megaman 4 ROM --- to 1F10 in megaman 3 ROM
32x32 block data (400 bytes): copy 4A510-4A90F in megaman 4 ROM --- to 1710 in megaman 3 ROM
Screen data (800 bytes): copy 4A910-4B10F in megaman 4 ROM --- to F10 in megaman 3 ROM
Screen Order (30 bytes): copy 4B510-4B53F in megaman 4 ROM --- to A10 in megaman 3 ROM
Screen Direction (10 bytes): copy 4B540-4B54F in megaman 4 ROM --- to A50 in megaman 3 ROM
Palettes (40 bytes): copy 4B5A0-4B5DF in megaman 4 ROM --- to A92 in megaman 3 ROM
Graphic/Tileset (1000 bytes): copy 2D010-2E00F in megaman 4 ROM -- to 52010 in megaman 3 ROM
and there we go.
some things are not going to be correct like palette animations and boss doors cause the formats are different for each game. i did not actually include those.
Unfortantely this is a whole ton of work as you can see
You cant just directly copy a whole level over and be done with it, gotta do it "1 piece at a time".
But if you do exactly what i did in this example, and you open Needleman level in MegaFLE, you'll see it's literally PERFECTLY copied ...the screen layouts, block data, all that important stuff.
(not sure why mm won't teleport all the way down, getting stuck at the top)