If the nuts come loose even a little bit the spacer will move and the studs will fatigue fracture and shear off, which is why people sometimes lose a wheel when running spacers because they haven't been able to check the spacer nuts are tight. Those using 4 M12 studs will have the 4 at an even spacing so the loads are even, using 4 of 5 studs is not the same, might get away with it, or might be more likely to get loose nuts and a loose wheel ...I'm not worried about the missing stud the because by design the nut cannot come off the stud even if it comes loose unless the wheel is removed first , and the stud can't screw off the nut because it's splined . Plenty of cars going about with just 4 M12 studs holding the wheels on and trailer wheels do not suffer anywhere near the forces applied to driven wheels .