Well, as far as s4 acting that way, I don't really know.  It's just speculation or guess on my part.  I will say that it was my most successful pct I'd ever had and so I've used s4 in every pct since.  In fact, I just bridge with it and seem to recover in my normal time frame.  Looking back I believe my doses tended toward 30 mg/day most of the time because of the way I measured it out.  i.e. my bottle shouldn't have lasted as long as it did.  Anyway, I think 30-50 mg/day is very helpful during pct.
Also, my current pct (5 weeks since last aas shot and had tapered down before that) is amazing.  I haven't lost a single pound.  S4 at 30-50 mg/day, ipamorelin and mod grf 1-19 at 66-100 mcg 2 x day, and for a few weeks using peg-mgf and igf1-lr3.  Have only used the igf/mgf for the last week or so- so most of the gainskeeping is s4 and the gh peptides along with high protein diet and good training.  I know that's a lot of shit to be on.  But, it's doing its job and then some.  And, the doses are minimal-lower side.  
I competed at 197 lbs in April, took 8 weeks off aas and held my weight (used gh at 2 iu and s4 at 30mg during pct), then, during my last cycle I went up to 225 in 8 weeks gaining 6 lbs of fat along with whatever muscle and water (which was minimal since I used aromasin throughout).  When I came off and went on the above protocol I would say I lost 2 lbs of muscle/volume and gained 2 lbs of fat (since I'm still eating like a pig).  Still, my lbm is up a whopping 15 lbs and holding/gaining and my bodyfat level is reasonable at 11% (measured again last night).  I'm now cleaning up my diet and losing a bit of that excess bf while keeping protein intake high.  
Long story short, I credit s4 as my gainskeeping base and I really look forward to trying ostarine if ever possible!