It probably depends on how you want to list the offer, I think.  eg As a variation so that you have 4 variations with a quantity of 1, and one variation with a quantity of 4, or as a promotional offer (which you can set up separately from the listing via Promotions Manager). 

I had the new selling form for a little while but then opted out, so unfortunately I can't take a look through the way it's set out to try and work out how to get to certain options or functions, but if by some chance you've been 'upgraded' to the quick listing tool instead of the new advanced form, look for a small link to switch the advanced tool at the top-right of the listing form.