I would think with some flexibility on dates you won’t have any problem getting four coach tickets.

Keep in mind different Star partners fly into different airports. The airlines with one-stop connections from IAD to Moscow are: Lufthansa, Swiss, TAP, Austrian (DME), Turkish (VKO) and Brussels (SVO). Probably best to decide what part of the city you want to arrive to (traffic in Moscow is abysmal) and aim for that option.