More PA-28's will carry more
Your best chance of finding an C172/PA28 capable of carrying 4 adults in a sensible and legal way is to look for an older (or old!), 180 HP PA-28.
But the C172 has its advantages, as Veryimmature points out, although I didnot think of "roomier" as one of them. The C172 is also a very forgiving aircraft with the lowest accident rate (together with the Diamond DA-40).