Thought I might add something that will speed up using the Twinmate with your M4-P in the field without having to remember to open up one stop every time you take a reading.
I use a Fuji GW670iii with a yellow filter, factor 2. I just set the iso dial on my Twinmate one notch slower than the ISO of the film I'm using. This works at all film speeds and I've tested the meter's readings against my M6 classic 2/50, that's equipped with an identical yellow filter, at all ISO settings.
Easy.