Add004 is simply lifebars. They come with a patcher .exe that adds these to your Mugen gameplay:
-Combo messages. (Think MvC2's YES, GOOD, GREAT, MARVELOUS...)
-A score system.
-A universal guard gauge, and guard breaks.
-A ranking system at the end of every match that keeps track of score/defense/first attack/tech/etc...
-Special hyper backgrounds and hyper K.O. backgrounds. (Requires slight coding knowledge.)
-A run sound and special dust effects. (Kinda minor, but it's there.)
-"Water" effects, like when a character jumps or is knocked down, there will be a splash. (Requires changing stage author name to "WATER", though.
-Just defending system.
-Last but not least, a tag system, complete with a assist. (Reqires knowing what state does what on a character, because some assists might involve characters taunting, just standing there, turning into projectiles/character-specific effects, vanishing, and other glitches.
Maybe there's some other changes, but the other stuff I've seen is very slight, or I haven't noticed any others.
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