Or get a mini mix of 4 cube !
It's not really cost effective to mix anything more than a cubic metre by hand, I have costed it both ways thousands of times, it's my job, ready mix all day long, depends on your area what they charge, expect anything from £70 a cube to £120.
4 inch (100mm) is enough for a double garage, PROVIDING the sub base is sound, and well compacted on good solid up disturbed ground, no made up ground, bust be a good bearing strata 100mm hardcore and 100mm Connie, job done, I love garage bases, knock them out every day.
If you decide to mix by hand you must remember,