Well, as a novice myself I dont know if I make more damage than help by jumping in here but a few things seems odd in your description:
mon-fri seems like a 5 day/wk to me
The +4 days restriction would if nothing else start task b on wednesday the week after (if you have mon-fri as workdays)
It seems like you have some other constraints in play, I tried the setup you describe and the SS+4 constraint works just fine for me.

My bad, I apologize. I meant Mon-Sat.

I wonder what other constraints might be getting in my way here