SFIV wasn't hard execution-wise either. Every installment of SF became easier in terms of execution. SF4 was easier than SF3 which was easier than SF2 - which was easier than SF1. If you have played every installment, this is very clear to see.
What was so difficult to do in SFIV? The game has "helpers" to help sloppy inputs be recognized.
then it speaks mountains of your actual ignorance when it came to the game. you're right that a lot of earlier sfs have a HIGHER execution floor (mostly because of no shortcuts and shorter buffers,) but i'm specifically talking about execution ceiling, particularly in regards to optimization.
edit: if you played the game at a very low/casual level, then yes, simply getting your special moves to come out is 'harder' in earlier sfs. but if you're talking about actually playing the game at a high level there's no contest to the differences in technicality.
what you're talking about is input recognition. yes, the older SFs were a lot stricter in terms of reading inputs, and thus requires more precise inputs than sf4 does (no dp shortcut, etc.) but SF4 is unanimously considered the most technical iteration in the series by far. c.viper's FFF (fierce feint fierce) into superjump cancel ultra is an example of something that's tough to do. hell, even just doing viper's FFF xx EX seismo into meterless FADC into ultra is still tougher than anything else that exists in the sf games.
there's also stuff like el fuerte RSF (run stop fierce) which required you to hit 1-2 unplinkable one frame links and you loop it like 5-6 times. like, if you wanted to go for it, sf4 has chars with a SIGNIFICANTLY higher execution ceilings than anything else in the franchise.
like, part of the reason the game itself was a lot more technical is because they used 3d models/hitboxes on a 2d plane. it allowed for very odd, highly technical combos because the pushback on some moves (on hit) would make it so certain follow-up moves would miss - but some moves could use this to time it so you could hit another move at the last possible active frame (essentially a meaty normal mid-combo) which means you had significantly higher frame advantage than normal and could thus do an otherwise impossible combo.
the few things that come close is like charge partitioning for urien in 3s, but even then it's 'easier' to get down because it's one particular maneuever that's re-used instead of long sequences of different and wildly paced inputs.
