I am in HS, but with input from my coach and various training methods I have put together a plan for your runner that is 50mpw. Just remember: I am a sophomore and I run 17:01 off of 30-35 mpw, and I believe that if your runner is not seeing results from that kind of training you may want to step down everything (both volume and intensity): 50mpw
Mon: 8mi easy (8:00-8:30) split into doubles if necessary
Tue: hard workout day 4mi of intervals + 2mi warmup cooldown
Wed: 8mi easy (8:00-8:30)
Thu: 4mi tempo (6:10-6:20) + 3mi warmup cooldown
Fri: 4mi easy (8:30-9:00) this is just to recover. basically a rest day
Sat: race day 3mi + 2mi warmup cooldown
Sun: 12mi longrun pace (7:30-8:00)
Notice the one track workout. Tell the runner to run that workout very hard, i.e., just below race effort. He should be very spent at the end. For the tempo run, the runner should feel satisfyingly refreshed by the end of the run. The tempo/LT pace is designed to steadily biold lactic acid faster than it can be disposed of, but the pace is also slow enough that it is not extremely taxing aerobically. He should feel a lack of energy in his legs and a slight burn, but not be breathing excessively.
My week of training looks like this (no doubles): 33mpw
Mon: 2mi recovery easy
Tue: 3mi intervals + 1mi warmup cooldown
Wed: 6mi recovery easy
Thu: intervals or tempo run 3mi + 1mi warmup cooldown
Fri: 2mi recovery easy
Sat: race 3mi + 2mi warmup cooldown
Sun: 10mi comfortably hard (7:20-7:30)
I want you to understand especially the small number of "quality" runs that I do. Elites generally do not run 4 hard wokouts a week. They run 2, maybe 3 tops. And remember that they are also doing 80-100mpw total, so the "quality" mileage is still a small percentage.
Read Joe Rubio's manual for 1500m training. You may think that it is for milers, but note that he says that 1500m runners start out as 5k runners, and that they gradually work on speed. Anyway, HS runners are so aerobically underdeveloped that a "5000 specialist" resembles a 1500m runner in terms of actual fitness.
To answer your question, do not add a 4th workout.