The diameter of those four holes is specified by the designator "8-32" which means standard screw diameter #8, threaded to 32 threads per inch. This is the usual way threaded holes are called out, rather than by saying the actual hole diameter in decimal inches and then saying the thread pitch.
So the machinist who has to make the part knows what drill bit diameter to use to make a hole and then tap it to 8-32 by looking it up on a chart, and the engineer who wants to buy the part knows exactly what mounting screws to use to attach it to another machine part.