I'm not a civil engineer, but do not just pour a new layer over the existing concrete. If you're going over the existing concrete, you would want to dowel in rebar to tie the new concrete to the existing.
You're better off breaking up the existing concrete, putting new stone sub-base, then pouring new concrete. 3-4 inches is too little ... I'm not sure how thick, but I would think 6" minimum ... but you check on that.