I have a foundations triple that has the seats in front of each other-found it very used but very cheap, so it was a good deal!
My only concern about the quad is fitting through the bus doorway...once you get it to the wheelchair/stroller section in the front with the seats that fold up you'll be fine, but it's so wide, I don't know that you could maneuver it through the door and past the driver ok.

I take the bus with my own kids all the time, but I don't take the daycare kids, as I don't trust them all to handle it atm, and the stroller issue-the triple would be too long to be able to maneuver on, I think. Also, we live in a small city with many bus "regulars" who detest strollers of all kinds-I've been criticized and yelled at for having a single stroller on the bus when my youngest was only a few months old. The drivers don't seem to like strollers much either, and certainly didn't step in when I was being told by other passengers to fold up my stroller even though there was no need to, so I wouldn't risk the potential problems by bringing a bigger stroller on the bus. I've seen a few people with wagons drag them on, and honestly, I can see why people get annoyed with a bigger wagon; everyone else has to squeeze by them to find a seat, and it's such an event getting on and off. I wouldn't take more than a double stroller on the bus, but that's just me and my VERY conservative city!