I'm not sure about 4-2 slip-ons for 350Fs, but there are 4-2 complete systems out there.
As for 4-1 sounding bad, dunno about that (or as a reason to rule them out). The 400F was the first stock 4-1 system and it came right from the 350F. 400F systems aren't that expensive used, and they bolt on to the 350F (although you've got to use the 400F pegs -- and they are getting rarer and more expensive -- and levers or rear sets or another fix). I've had a stock 400F system on my 350F since 1997 and I sounds just fine -- better yet, it functions just fine too with only minor tuning to the carbs and using stock jets.
I've never been a fan of Mac systems, but there are other options out there. You get what you pay for, I guess.