first of all, yes, ms4 just pales in the huge shadow of ms3, i agree. second, ms4 uses just too much recycled backrounds. the games, of course, used recycled sprites too (soldiers, in every game), but still, ms4 is just super-rehashing even of ms1 backdrops. third, not much actually new stuff. enemy slugs suck balls, they dont have invulnerability time after they are hit, and that metallish system, etc, alot of small engine changes which one person pointed out perfectly - the motto of ms4 was something like "fuck these crabs and all that non-human shit, let's get back to good ol' ms1 type war!" but since they used ms3 engine, where soldiers before falling can "hog up" player's bullets (instead of ms1 where once soldier is dead, bullets fly thru the sprite) and that is the main problem, because - another bad thing - the screen "stops" for too many times! and tons of enemies constantly respawning... earlier games didn't abuse all that stuff i mentioned. plus, the music, while everybody like it, is different, the ms music was always something really cool, funny, sort of mix of orchestra and rock/metal, it even had distorted bass! ms4 has very different one, which is good, no question, but lacks "ms spirit".
and the most important thing - the game just feels "RUSHED". alot of things were left out. not much in MS4, but shitloads of cool stuff was cut out of MS5. ms5 was much better effort, though it also just SCREAMS "RUSHED!".
my opinion is - if they could sorta combine MS4+MS5 into one game, that would be something MUCH better. but instead we have two half-assed rushed titles. too bad.