I'll throw my vote behind a taxi. The cost is $45, plus toll ($4.15), plus tip, so $56-$60, depending on how generous of a tipper you are. I find catching a taxi at JFK so much easier than a car service; you walk outside and get in line, as opposed to having to locate you car, which can be an incredible pain. Plus, the car services that are priced to be competitive with cabs are unreliable in my experience. (Note that there appears to be a fairly even split among New Yorkers on this board between those of us who prefer a taxi from the airports, and those that prefer a car service.) The bus, even if it were an option, doesn't offer any savings for a party of four. You can save some money using public transportation -- which ordinarily I would recommend -- but given the size of your group and the hour, I'd just want to hop in a cab and get to the hotel with minimal fuss.

By the way, I recommend the Parker Meridien over the Hyatt.