Been running some numbers through Druid Wiki calculator to see what pieces to use in a max dps set. Mainly I was checking with regard to 2T4 bonus. Some results:
5500 AP
50% Crit
142 hit
PS: 2
BoK
Teron
Differences between T4 and Top gear in the slot:
Helm: 200.97
Chest: 246.30
Hands: 225.26
Shoulders: 155.86
Legs: 324.12
Helm + Shoulders: 356.83
2T4: 419.79
7000 AP
65% crit
142 hit
80 expertise
100 Haste
300 ArP
PS 2
BoK
UR
Teron
Differences between T4 and Top gear in the slot:
Helm: 200.13
Chest: 246.14
Hands: 223.54
Shoulders: 152.22
Legs: 320.17
Helm+Shoulders: 352.35
2T4: 389.54
The two different runs I did were just to check scaling. The differences are pretty negligible between two sets at vastly different gear levels so the following conclusions are probably pretty accurate regardless of your gear (as long as its near the level to obtain these things).
At absolute top end it seems Helm + Shoulders should be T4. Boots, Belt, Bracers and Gloves should be T6. Chest and Legs should be the sunwell pieces.
Some of the sunwell gear (notably the helm) may not be easily available but this doesn't change the conclusions (since the helm is one of the pieces you should use T4 on). The only exception here is if you are a non-leatherworker. If you are non-leatherworker you cannot make the
[Carapace of Sun and Shadow] so if you cannot obtain the Sunwell Chest (which probably drops of Kil'Jaeden) your next best piece is the S3(!) chest. As such it would be much more reasonable to use the T4 chest and T4 shoulders as your 2T4 pieces for the bonus and use a Cursed Vision, Tier 2 Eng helm or sunwell helm in the head slot.