All plugs are grounded. The difference between 3 wire and 4 wire has to do if you want to include the neutral or not. 3 Wire = hot, hot, ground (hot to hot = 240VC / Ground = safety only). 4 wire = hot, hot, neutral, ground (hot to hot = 240VAC / hot to neutral = 120VAC / Ground = Safety Only). You want to have the 4 wire connector if you are going to use a 120VAC circuit (like a light) on the machine.
I have been standardizing across the shop to the L14-30R/P connectors so I can move things around. R=Receptacle and P=Plug. This is a 30A plug and is sometimes overkill for the 20A 240V machines but that is allowed. This twist-lock is a lot more user friendly than the dryer/oven style 4 wire plugs.