Training 4 miles 3 times a week is enough to get you through a 10K in reasonable comfort. You won't set the world on fire but you should be able to finish. I'd suggest you move more of your running outside, and make sure you're doing at least those 4 miles 3 times a week. The main thing for now is to be consistent with that. After a couple weeks you could add a 4th day, start short, maybe 2 miles, then add a mile each week until you're doing 4 days at 4 miles. Build from there.