4 miles would take about 15-20 minutes, I suppose. If you go in at a regular time, there are the 980 buses in the morning. It's rarely longer than a 10 minute wait for those. It's about a 25 minute ride to West Falls Church Metro, 5 mins to change and a 10 minute ride to Ballston. The whole commute should take about 1-1.25 hours. I manage to get through some recreational reading in that time.

For about $3.30 a month for the locker, it's worth having the option.