I have never ridden one, but I'd imagine they're to a tandem what tandems are to single bikes. Problem is that it probably would get even less use than a tandem since you need 4 to fill the seats. Borrow some riders from the local club, and I think you could have a lot of fun / scare yourself a lot depending on your nerve and love of speed.
I would imagine this means probably scary fast downhills, riding over rollers like they aren't there but slogging up longer hills as unless your name is Phinney you're unlikely to have 4 cyclists in the family who are all up to enthusiastic single bike rider standard.
I would be concerned about braking and control - kinetic energy increases in proportion to mass and speed squared. This measn that the quad will put more more than double a tandem's stress on a brake system as it is twice as heavy and a bit faster. However most quads I've seen pictures of seem not to be more fun ride than long downhill oriented, so get by with reasonable rather than excellent braking and slightly more planning ahead from the pilot, exactly as one needs to when driving a vintage car.
Maybe when I am rich I will get a triplet and a quad for taking friends for a spin?
Great people and helpful as can be. My wife and I live in Manhattan, KS (about two hours to the west of KC) and took our tandem to Mark a couple of weeks ago for its first check-up in about 1700 miles. We also bought our tandem talk from them. Very friendly. If you ever need advice, he's the guy to call. I once called with a question and caught them walking out of the house on Labor Day weekend for a trip. Mark took the time to andwer my questions even though it no doubt delayed their departure.
It was a hoot to see them pull up at the end of the day on BAK.
From another forum and discussions of multi rider tandems this piece of info
>Before they started using things with motors, track riders were paced
>by multi-rider tandems. (One thing Orient discovered: that after 5
>adding more riders doesn't let you go any faster)