I trying to get some replacement lock nuts for my RC truck. When I measure the axle shaft it measures at 5/32 which I know is 4 mm. But when I look at the 4 mm lock nut required tool a 7 mm wrench. And the nuts i have on the truck use a 9 mm wrench. So What size lock nuts should I get? Would I be fine buying the 4 mm lock nuts?

Buy them both. You can't go wrong. We're not talking about big bucks here right. Matter of fact, throw in a few other sizes also.