My concern is: could this be harmful to their coats for only bathing 4 times a year?
No. As long as they look and smell reasonably clean, coats are shiny and you're not seeing excessive itching or skin flakes, it shouldn't be a concern. Keep their coats brushed out and you shouldn't have any problems.
I have to tell you though, once the shedding starts you are probably going to want to bathe them more often. Most of my GSD clients like their dogs groomed once a month or so, just to keep the shedding under control.