Hi David
looking at the spy shot of the slim 4 and from the non tub chassis S4'S i have seen trackside i would yes they do fit but they would obviously need to be held on by body post, you can get the bodyshells from atomic carbon , pm jonathan ( Oldtimer ) on here he will sort you out
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[ and does the tub chassis fit it.....??? ] if you mean can you replace the old chassis for a tub one then i dont see why not , jonathan is the man to talk to but i'm sure some of the other AT guys will help you out also.
Neil..