what year s4? i really really doubt you're telling the truth, either that or the s4 didnt have good snow tires, which will make all the difference.
saying that because quattro is easily the best AWD system, i used to have a B5 s4, it was a great car but unreliable as hell, anyway, i got places in mine on worn out snow tires that my friend with new studded snows couldnt get in his outback with a shit ton more ground clearance.
if its a newer s4, i dunno, it might suck, i know they are doing all sorts of retarded electronics and shit.
ive driven a lot of different audis and subarus, a b5 s4, b6 s4, b5 a4, audi 90 and a c4 s6. all but the b6 in snow. and a wrx, sti, outback xt, some older outback, and a 2.5rs and all but the wrx in snow.
every time ive been in an audi in snow its amazing, just an insane amount of grip, we were launching the s6 from a stop and you would only spin the tires for maybe 2 seconds before it would grip up and give you literally no wheelspin, and that was a damn fast car too.
subaru's are ok, definitely noticeably lacking in all out traction, and a definite disconnected, unresponsive feeling front end when you turn in in snow. not a bad awd system, but just nowhere near audi's.
plus its just pure mechanics in audis favor, torsen diff vs viscous coupling, that and subaru uses a lot of electronics to control the awd system while quattro is for the most part, mechanically controlled.
one of my best friends is a subaru tech and he even admits to audis superiority.