I simmed this out a couple weeks ago but I suppose a re-run of the simulation could prove useful:
Using Patchwerk Style Fights:
Priest_Shadow_T11_372 : 24006dps
Code:
DPS DPS% Count Interval DPE Hit Crit Max Crit%
shadowy_apparition 1049 4.4% 29.9 16.43sec 17548 1 13259 26536 30184 32.3%
Priest_Shadow_T11_372 4-Piece Disabled : 23769dps
Code:
DPS DPS% Count Interval DPE Hit Crit Max Crit%
shadowy_apparition 809 3.4% 30.0 16.28sec 17548 13482 20416 23218 32.1%
This leads to a DPS increase of 237 or ~80 Intellect.
Helter Skelter Style Fights:
Priest_Shadow_T11_372 : 21305dps
Code:
DPS DPS% Count Interval DPE Hit Crit Max Crit%
shadowy_apparition 1538 7.2% 43.9 11.21sec 17497 13234 26480 30184 32.2%
Priest_Shadow_T11_372 4-Piece disabled : 20966
Code:
DPS DPS% Count Interval DPE Hit Crit Max Crit%
shadowy_apparition 1190 5.7% 44.1 11.16sec 13482 10180 20368 23218 32.4%
This is a difference of 339 DPS or ~114 Intellect.
Conclusion: I would expect the DPS increase on these to be even higher in practice. Therefore, I believe it is a good idea to go with the 4-set bonus.