Yep, my bad, 4 to 7. Thanks Elric.
Offense loses 4, just like best defense.
So, 4 for offense, 4 to 7 for defense.
Given this, the OPs solution is not that bad on the surface.
However, I do think that with the sheer number of powers that PCs acquire over levels, that the solution should not be as great as the decrease. There are many ways for players to boost offense and defense, and shy of that, heal, etc.
My solution:
+1 to hit and +2 defense (except AC) at Paragon
another +1 to hit and +2 defense (except AC) at Epic
All PCs can use their primary stat for their AC boost in light armor.
The new PHB II offense and defense feats are not allowed.
AC is even screwier than the other defenses. The lowest AC is 8 lower than the highest, but it adds a 0 to +5 stat on it at first level. And heavy armor adds another +6 when going to masterwork that light armor does not. Any class not in heavy armor that does not pump a high Dex or Int every single time is screwed. A sorcerer might as well just throw in the towel and start buying armor proficiencies and get to Chain.
1st level Sorcerer that does not buy armor proficiencies: Dex (or Str) 16 (maybe) = 13 +4 (pumping every level) +6 magic +15 half level = 38.
Without armor feats, this Sorcerer is screwed already. The best level 30 soldier (like an Ancient Red Dragon) should be +37 to hit AC and even creatures like Orcus with +37 and Tiamat with +39 are going to hit the 30th level Sorcerer on a 2. That really is unfun for a player. Once people find this out, very few people are going to play an Epic level Sorcerer. Even their occassional two AC boost spells are not enough. They are forced to take Leather at a minimum.
On the other hand, Godplate and Heavy Shield = AC 47 at level 30 without armor or shield specialization, AC 49 with. These high level foes only hit the Paladin about half of the time, slightly more than that if he uses a two handed weapon.
But, I was unable to come up with a good rule to help Sorcerers beyond allowing their primary Cha stat to affect AC. At least they would be on par with Wizards that way.
Armor at high level with a stat of 26 (without magic or anything else) works out to:
Cloth: 2 + 8
Leather: 4 + 8
Hide: 5 + 8
Chain: 12
Scale: 13
Plate: 14
There is no clean way to boost the Sorcerer without messing with the balance here.