Ah yes, the dreaded decimal point, sorry!
I don't agree that a plumber's tube cutter would have problems with that 4 mm wall - it's only a question of keeping on adjusting the cutter wheel inwards after each turn of the tool. It may take a bit longer but I bet it would do the job, after all, plumbers etc often deal with copper tube of 1 inch dia and up, and that must have wall thickness of at least 3 mm.
And the plus/minus length tollerance shouldn't be a problem either. Perhaps a couple of practice cuts on something else to get used to exactly where the cutter wheel is in relation to the edge of the tool, but then marking up the tube with felt tip pen plus a SMALL notch with a triangular file made with the tube on the bench before taking the tool to it should do it. At 50 mm lengths a vernier caliper would happily get you there without too much bother, if you don't trust a steel rule - just a question of going slowly/being careful.
My thoughts FWIW.
AES