As a big fan of FNR3 I delayed buying FNR4 because I knew it would lead to FNR3 ending up on the shelf gathering dust. I didn't want that to happen because it was my favourite game! I relented when I saw FNR4 for under £20 on Play.
Initially, I was not impressed with the modified FNR4 controls. They didn't seem as instinctive or responsive. However, after a few fights I'm now used to them and think the boxer handles more realistically. It was too easy on FNR3 to throw a series of haymakers and destroy your opponent - you can't do that on FNR4, so fights are more competitive. Counterpunching and your opponent's A.I. has also been improved - you can't get a cheap win anymore.
The online has also been improved; your boxer now always fights from left of screen - so it's easier to find an opponent and don't spend ages with people cancelling on you (which was the worst thing about FRN3 online!)
You are also able to download free user-created boxers; I've had Naz, Eubank, Benn, Khan, Rocky, Apollo Creed and Ivan Drago so far! You can find them by "highest rated" and "most downloaded" to make sure you get the best version - as there are often several versions of the same boxers.
Graphically, the game has improved and some of the glitches have been ironed out. The boxers are more solid and there haven't been any instances of punching straight through an opponent!
Highly recommended game. A lot of improvements which did not seem apparent at first. Recommended to FNR3 players who are skeptical and have read mixed reviews.