Having played two and then three upon release and looking back retrospectively, the excitement of 4 will be hard to deny considering it's the finale. If you can genuinely only afford one, go with the new one as that's been one of the best reasons to own a ps4 for two years now. You can always go back and play the other games, but the buzz of hype will only exist for a period of time around U4 and sometimes being a part of that really adds an unconditional value to the experience.