I have both species on my land. At this time of year Snipe are commonly seen and heard all day long, usually in flight but also on the ground or perching on a fencepost, while Woodcock are secretive and are heard much more often than seen, and then usually in early morning or late evening. These birds are superficially similar, but as Longwalker said, they are completely distinctive from one another in flight. Even on the ground, the proportions of legs, bill and head are quite different.
If you saw four together at one time, especially in a slough, I'd bet good money that they were Snipe.