Hard to keep 4 pitches sharp. Big leaguers have been throwing their pitches for years and can afford to cut back on one pitch to develop a new one. In any case, if one of those 4 isn't working well he won't end up using it very often so I wouldn't worry about it. Sounds like he's doing the right things.
Most pitchers need a fastball (usually two and 4 seam), a breaking pitch and a changeup to be successful. If they have two very strong pitches they can sometimes get away with two pitches.