If it's grayscale, then providing it is just shades of k(from cmyk) then you wouldn't need four colour, if it uses a spot colour red and grayscale it will be two colour and therefore cheaper than four colour.
However....
spot colour would be pointless with digital printing since it relates to a colour pre-made that would go into an ink chamber of a printing press. But if it is a short run, say less than 500 a4 sheets then it'll still be cheaper to go digital, decent quality digital is a possibility.