The fuel pump relay has an rpm writen on it. That is the cut-out point. This is usually around 6000 for 4cyl and 6cyl, but the relay doesn't count revolutions it counts coil firings. The 6cyl will have 50% more coil firings than the four cyl at any particular speed.
In other words 6000rpms is 12000 coil firings a minute for a 4cyl and 18000 for a 6cyl. Put the 4cyl relay in a six cyl and it will cut out at 12000 coil firings/minute probably 4000rpms.
Bosch Master, ASE Master, L1
33 years MB technician