Having spent a lot of time on tanks, I would argue that pursuing technical solutions that don’t account for the human dimension of sustained ground combat is a mistake. The four-soldier crew gives flexibility that three cannot.
Pulling local security is a real requirement inside a tactical assembly area. Doing so while maintaining weapons, conducting maintenance, eating, and getting some sleep is already tough. When in radio listening silence you need a runner, and during tactical road marches you need an air guard.