4 took place in a completely different sort of environment, with a whole different story, with an online co-op mode for the entire story that 3 didn't have. There are also smaller mechanical differences, but I mean Ubisoft does run formulaic franchises, so maybe it's not enough of a difference to please somebody who's just not as interested in the open world FPS setup from the ground up, but saying it was "an exact copy of 3 with slightly changed graphics" is such a biased and reductive take that it gives the impression that you've already made up your mind before you asked the question and that nuance isn't something you're looking for.

I suppose the answer is, that premise is interesting enough to some that they didn't require as much of a drastic change to still play more, it's more core to what they might enjoy, whereas Far Cry for you is more tertiarily enjoyable and you'd need more differences between them (such as from 2 to 3) to try a new installment, compared to a core fan that likes whats already there and wants minor improvements and tweaks to the formula to freshen up the experience they already fundementally enjoy and want more of. To that fan a drastic change in setting and scenery with some mechanical changes to increase immersion and mix it up would be enough.