Combat Casting is a very useful feat.
There are several situations in which stepping 5 feet back is just not enough to avoid a Attack of Oppertunity. You may be surrounded by multiple foes, one or even several foes have reach (weapons) and so on.
+4 nearly negates the effective +5 to the base DC.
Remember, if you get hit by an AOO, you have to beat a DC based also on the Damge (which will soon become more than 5 points, which is the Base DC Difference to Casting on the Defensive), and you roll only to avoid loosing the spell - you suffer damage instead.
Combat Casting and steadily having max Skill Points on Concentration may make you effectively immune to AOOs due to Spellcasting, but if you don`t do that...

Avoiding AOOs is important for a d4 hit diced wizard or sorceror.

Even - or better especially - in the rare circumstances a spellcaster is confronted with AOOs during Casting, he cannot afford losing hit points...
Though, obviously, Combat Casting would lose most of its use when skill focus grants +4 on a skill check. (The Feats stack, I think... Unnamed Bonuses).
With +3 it is fine.
For deciding between +3 and +4 remember the magic items rules.
A +3 is worth slightly more than 2 +2s. (3 x 3 = 9; (2 x 2) x 2 = 8)
So +3 is "in line" with the usual calculation rules for bonus to skills, abilities, and attack rolls.
(Hmm. How about Luck of Heros compared to Iron Will, Great Fortitude and Lightning Reflexes: 3 x 1 = 3; 2 x 2 = 4. Seems to be fine. Hey, Luck of Heroes is even weaker, its a named, notstackable bonus type!)
Mustrum Ridcully