Honestly, CV IV's weaknesses have a lot to do with it being an early SNES title with limited game data being used and Konami wanting to make it darker and "more realistic". While I'm not sure I like Simon's new look, I really do like the atmosphere. I think it is the creepiest of the CV games. The music and graphics design really create a dark, spooky environment that I think works well for the game.
Rondo has a very different kind of appeal, quite frankly. Sometimes I think the game is way too light-hearted, and a couple of the CD tracks actually bother me a little because they're not serious enough.
Yet, both are great games that are fun to play even today, and both are definitely CV games and will never be mistaken for anything else.
your first point may be valid, but why did the 2nd one the SNES so baddly too, compared to rondo? it even had a lot more of memory than the first had (sure still no real comparions to a full CD of data, but which most of it was used for cda red book anyway) and came even out two years after rondo.
also as said, the music in CV4 was great and added a lot to its darker atmosphere. though, the graphics are not really dark per se, more like muddy. like the SNES tried obsessive to pull off darker colors but for which it wasn't really designed. kinda of replicate the MDs brown palette, but with a higher color count.
I must say, rondo has its brighter parts compared to CV4, but it also has way muuuch more darker and creepier locs than anything ever seen before.
few locs that come to my mind:
1. the saurus in the 1st stage walking in the dark background before entering the castle (only red eyes)
2. the heavily splattered saurus is chasing you inside the castle (heart attack)
3. the beginning of 2nd stage with the great thunderbolt effects (enlights the dark back ground)
4. the sitting man that falls into dust when touched or whipped against the wall.
5. particulary every boss location is way more creepier (and adrenaline loaded) than anything in CV4.
-> especially the by bones made skeleton boss comes to my mind. damn I was really terrified while fighting against him the first time. such a creepy loc with all the bones and skulls everywhere and then suddenly the boss rises out of it and even transforms itself several times. only creepier in a similar way was the boss "legion" in SotN.
6. also that part (in the 5th stage I believe) with the graveyard where those dead souls try to grab you with their arms from the ground and drag you to hell. or the picture on the priate boat as mentioned before, as well the mirror etc etc... oh and the landscape with the viaduct in the back ground, where all sudden a mid-boss is coming out of the behind. that one dark landscape picture alone brings me more creepy atmosphere than almost any stages in CV4 combined.
I could go on and on...
walking throught CV4 is just like whipping the shit out of the same repeating as well mostly boring enemies again and again.