+ aquiring 4 piece is regulary easier then hoping for certain drops
- only real downside is you'll end up lower on pure mastery when going for 4 piece
If you mention shoulders from Alysrazor you shouldn't ignore Shannox trousers. If you're interested in upping your overall avoidance values those items and the tier chest are of high value. In BiS gear you can wear Offhelmet, -shoulders, -chest and -trousers and still sit at 102.4% combat table coverage with about 33% avoidance combined. Or you could go for 4 piece, sitting at about 1% less avoidance combined passively but trump that with the setbonus 1/3 of the time.
tl;dr - 4 piece is pretty good with any gear combination and regulary easier to accquire, go for it (well unless you trip about several offpieces on your way that'd get shared otherwise).