I don't see any good reason to think an artificer doesn't have a caster level equal to his class level. It doesn't say that he does, but neither does the wizard class description. It says he is not a spellcaster, but it also says that infusions function just like spells and follow all the rules for spells. I would have said the caster level mechanism was part of those rules.
I am leaning towards following the Bonus Feat Precedent of not requiring Prerequisites. Would this post a Balance Problem for any of the Options?
Attune Magic Weapon - normally level 5, you could get it at level 4. Doesn't seem too bad.
Craft Construct - normally level 5 (due to arms and armor prereq), you get at level 4. Not too bad.
Exceptional Artisan - no significant prereq, no problem
Extra Rings - normally level 12, you get at level 4. Sounds bad, but you probably wont be able to afford more than two rings until you're getting close to level 12 anyway. (Unless they are really wussy rings like featherfall etc, but then who cares?)
Extraordinary Artisan - no significant prereq, no problem
Legendary Artisan - no significant prereq, no problem
Wand Mastery - normally level 9, you could get at level 4. This one seems like the worst to me... the feat is (IMO) meant to make wands useful into higher levels, and risks making them broken at lower levels. Letting a 4th level character upgrade their wand of fireball from CL 5 to CL 7 and upping the save DC seems bad.
So of all of them, I'd say only Wand Mastery raises flags with me.