So there have been a lot of papers looking at prosauropod stance, but not specifically on Riojasaurus. Still, aside from believing in my mad skeelz as a skeletal drawing-er, bipedalism is pretty well bracketed in Riojasaurus. Papers by Bonnan & Senter and later by Mallison showed conclusively that the more primitive Plateosaurus was clearly bipedal, and later papers looking at the more advanced Massospondylus and Adeopapposaurus, and even the quite advanced Aardonyx all show they were bipeds (with the latter making some changes to the hind feet to shift towards graviportalism even before the switch to full-time quadrupedalism!). It doesn't look like you get quadrupeds until you get way up to melanorosaurids at the base of Sauropoda.
So what gives with Wikipedia? Well Bonaparte described it as an obligate quadruped originally, and the only two attempts at a skeletal reconstruction since (Peter Galton and Greg Paul) both restored it as a quadruped (though Galton's doesn't look terribly comfortable in the pose). So without an explicit paper Wikipedia's editors presumably just didn't change anything. But from a phylogenetic bracketing case it would be surprising for Riojasaurus to have been quadrupedal (same with Yunnanosaurus, or any of the massospondylids like Lufengosaurus).
I wasn't married to the idea either way of course - it's not crazy that there could be more than a single origin of quadrupedality in basal sauropodomorphs (Anchisaurus may or may not end up being an example of that), but the bones didn't support a quadrupedal animal, so the above result is what you get