We got our first tandem in 1994 and our triple in ~2009. I really didn't feel much of a learning curve moving from the tandem to the triple. Partially because 1) We were pretty experienced with the tandem, including loaded touring, and 2) We pulled a Burley child trailer behind the tandem for four years before we got the triple. I found the triple way easier to captain than the tandem + trailer. Not sure about going from a triple to a quad, but I don't think it would be much of an adjustment. We have done loaded touring on our triple, and apart from the extra balance needed when starting out with the extra weight of the bags and stuff (which probably would weigh as much as added small child), it wasn't that big of a difference. The hardest part in general is being in tune with how much bike you have behind you, particularly when passing, making turns, going around obstacles, etc. Again, to me it wasn't a big adjustment. Apart from the initial learning curve for our son (he was five when he started on the triple), we didn't really have any of the issues MikeandJean mentioned. Honestly, we loved riding it together and had amazing family times on it. Truly one of the best investments I've ever made!
Transporting the bigger bike is probably the biggest challenge. Our Santana Cabrio fits inside our Honda Odyssey with only the front wheel of the bike removed. Super easy. You could also roof-top it. Or if a coupled bike could split in half to transport.
We're thinking of selling our Cabrio this spring or summer, so if you are interested send me a direct message. (It's hard to part with, even though we rarely use it now that our son is 15 and a strong biker on his own, thanks to the time spent on the triple with us.) Medium frame with "v" type top bar for better standover. 700c wheels. S&S couplers. 27 speed Ultegra/XTR. Etc. Good shape. I'm in eastern Pennsylvania.
Santana can make an extra section to turn a Cabrio triplet into a quad, by the way...
Last edited by Philly Tandem; 02-28-20 at 08:17 AM.