Can't say for all brands but with Yami it's pretty easy - all the four strokes have an F in front of the model name. F225, F150, F115, etc. Not always on the hood but definitely on the sticker on the mounting bracket. Honda is all four strokes, period. Suzi pretty much now too.
Keep in mind your gonna get a lot more hours out of a maintained 4-Strk than you will with a comparable 2-Strk.