i've been interested in getting a puerto rican cuatro recently, as it's basically like a mandolin with an extra string (different notes, same intervals).

No it isn't - it's tuned in fourths, B, E A d g c' (the lower strings octaved as on a 12-string guitar, and I use a 12-string guitar set on mine).

It's more like an ud with the lowest course missing, or a cobza with an added lower course. (Sounds much like a cobza).

Mine is a cheap Far Eastern one but it sounds very good. The main problem I have with it is that the string tension is mandolin-like - compared with an ud, cumbus or cobza it's very hard on the fingers. And the tailpiece is a cheap and nasty bit of stamped sheet metal. (Also I don't get on with fretted lutes as well as fretless ones, but that's negotiable)