Would you say 4m is worth it for him? He's 26 now so there's still plenty of time to squeeze out something.
unlesss
pardew wants to play him at dm
His main issue is that his posititional sense is horrible, and decision-making when to make offensive runs. It has always been bad but with Pellegrini it's record bad.
Mancini never let a full back go too far up. Richards was then limited to a shorter area to cover, and rarely ran up, so he was OK. He's quick so even though he loss his opponent a lot, he often regained him.
But with Pellegrini, the full backs have an immense amount of freedom. They can be anywhere. Problem is that Richards suddenly make these stupid runs and when City loses the ball, he suddenly finds himself in no mans land and everyone goes "WTF is he doing up there?". He could be down at the corner flag, higher than anyone in midfield. Then when the opposition gets the ball, he jogs down and is basically at the midline when the opposition is at the penalty area. Constantly, several times a match.
I guess he won't be that bad if you just put him in some kind of leash. His pro's are speed, reactions (in 1v1) and strength. However, positioning, decision-making and crossing are pretty bad. He can't cross, at all, just along the ground.
edit
£4m is probably good value, as long as you don't give him the insane wages City gave him, £~75k.