Weapon Focus gives you a +1 bonus to all attacks with a particular weapon. Dodge gives you +1 to AC versus a particular opponent. The key number is +1, and there are limitations on the bonus.
Deft Opportunist gives you +4 to attacks of opportunity, and to be completely analogous to Weapon Focus there should be a limitation on its use; the bonus is to AoOs incurred when moving, or something (making it ant-Mobility). Mobility gives +4 to AC when your movement provokes an AoO. The key number to these AoO feats is +4 and it seems that there should be a limit to the bonus; Deft Opportunist might be a little too good.
Anyways, your feat should grant a +4 bonus to AC against attacks of opportunity, but there should be a limit on when the bonus applies. If you make your feat analogous to Dodge, make it so you have to specify the opponent the bonus applies to. Dodge and perhaps Mobility could be the pre-requisites.
If you make the bonus apply to everyone, it should be less. +2 would be low enough, I think. Combat Expertise and maybe a few ranks in tumble could be pre-requisites.