Dzeko is 4th choice striker because Mancini having seen him in training and games believes the other 3 are better than him. Most fans would agree Tevez and Aguero are better, I think Dzeko would concede this too. The probelm is when Ballotelli gets the nod in front of him and this was exactly the same probelm Tevez had.
I personaly prefer Ballotelli to Dzeko, but his behaviour off the pitch would get him suspended for 4 or 5 games a season at any other club and his behaviour on the pitch has been an unaceptable. He behaves like a bid child and yet seems to get away with it all the time, Dzeko behaves like a good pro and for this reason I would play him before Ballotelli until he has a change in attitude.
Dzeko does need to improve his performances, he is not worth the 27mill we payed for him, his touch and his positional play is poor, he is lazy at times and you have to question his mental strength when he's complaining about being on the bench and not fighting for a start.