You only have four smallish fish so you don't want to serve very large meals. Feed only as much as they will eat every little bit in about five minutes or so. If you see bits of food settling out around the base of your rocks and your fish are no longer hunting for the food, feed less next time.
That would be one pretty small pinch of flake food or a little piece of frozen mysis thawed out in some tank water. It's kinda hard to say how much mysis to give, I don't know how the stuff you have is packaged, in a flat, or large or small cubes?
I have two pink skunks, a tail spot blenny, and a small yellow wrasse. I give a wee little pinch of flake food, just a smidge, once a day and some small portion of frozen food once a day, if I feed mysis I break off a piece from the frozen flat pack that is about the size of a nickel, thaw it out in a cup of tank water and dump it in. So they get two meals a day usually.
You can tell if you are feeding enough by looking at the condition of your fish, they should be plump in the belly and head. If you notice they are looking flat or thin behind the eyes or in the belly you need to feed them more.