At 4cm, it isn't a rapidly growing young'in anymore. If it's not looking overly skinny, it should be fine with 3 or 4 crickets every other day. I probably wouldn't feed it full on adult crickets, a couple of molts shy of full size would be fine for a 4cm frog. Overfeeding doesn't seem to be a big problem with my Grays (they do seem to stop eating on their own), so feed more if you like.

You can also feed it other items for some variety, earthworms are particularly great nutritionally if it will take them (you can collect them from a chemical free area). If you plan to release it, there will be no qualms about other wild caught insects either. It's probably too early for moths, but it's house fly season where I am and my two frog guests gobble them up with entertaining acrobatics.