I have a Hollywood rack that fits the Class IV receiver on my pickup. It's meant for three, but I can get four on it. I'm pretty sure they make one for four as well. I got mine through Scholl's in West View. I'd bet just about any LBS can order the right one for you.
(The only downside is I can't lower the tailgate while the rack is on - although I have seen racks like this that fold down with a release pin to open tailgates.)