I think the best changes going to reforged +4 from +3 are the +40 to evasion and +25 to the magic evasion, as well as a huge boost in accuracy in pieces that had no accuracy in their +3 versions. This also includes huge boosts in pet stats such as magic accuracy in the +4 reforged piece when it had none in the +3.
Otherwise is seems accuracy is +5, and attack (only if present on the +3 version) is +10. Stats are +5 but not across the board - some stats have no increase whereas others that do increase always increase by +5. Pet accuracy, attack, and MAB seem to show a similar marginal increase. I did notice the BLM AF armor received a 25% MAB upgrade from having none in the +3 which is kinda cool for blm.
What's
kind of poopie is that Artifact Armor Magic and Melee skills show increments of +2 from reforged, to reforged +1 (or starting at reforged +1), then reforged +2, then reforged +3, but an increment of only +1 going to reforged +4.
An artifact armor example:
WHM AF feet Enhancing Magic Skill: Reforged = 15; reforged +1 = 17; reforged +2 = 19; reforged +3 = 21. So +2 increments. This only increases by +1 to reforge it to its +4 reforged.
MNK AF legs Guarding Skill: Reforged = +20; reforged +1 = 22; reforged +2 = 24; reforged +3 = 26. Again, +2 increments. Guard skill increases by only +1 in the +4 reforged.
A relic armor example:
WAR Relic Head Parrying Skill: Reforged = 15; reforged +1 = 17; reforged +2 = 19; reforged +3 = 21. So the same trend as artifact armor melee skills. Again, only an increase of +1 to parrying skill going to the reforged +4 head piece.
SMN Relic Hands Summoning Magic Skill: Reforged = 17; reforged +1 = 19; reforged +2 = 21; reforged +3 = 23. Going to reforged +4 is only an extra 1 skill.
What's
REALLY poopie is that there seems to be no changes to other augments on pieces, some of which are job-specific, such as gear haste %, Magic Burst Damage bonus, Critical hit rate, enhancing magic duration %; cursna effects; Blood Pact Damage %, fast cast %, etc.
I think people are most disappointed with having the minimal increases to skill and most of all, none of the other augments increase at all (that aren't stats or acc/att macc/matt.