Yeah, you definitely need larger prey. At L4 they should be eating house flies, blue bottle flies, or small roaches. Hydei and melanogasters are too small for most L4.
Way back when I was doing this I would place the trap in a clean bag and put them in the freezer for several minutes before dumping out the stunned flies. I'd pick the best flies and toss out the small flies... unless I needed them to feed young nymphs. It worked for me, but I no longer use wild...