From what I understand, they are regular Towncars, so for a party of 4 people, one person would have to ride in the front. I don't think they'd allow you to all "squish" in the back, they want everyone buckled in properly.
We've used Tiffany Towncar's stretch limo a couple of times and it worked out great for us. But for our cruise next summer we're going to save a few $$ by requesting their van which holds a lot more people than just 4. (I don't want to give a number, because I'm not sure, but I know they hold at least 6-8 people). It's DH, 2 big teenage DSs, and myself, and we'd rather have the extra room that the van will give us, than be in the towncar. If DSs were young, we'd probably do the towncar though.