It is understandable if the modelling movement started in a country using the Metric system making scale models of locomotives made to imperial dimensions.
They may have had the original drawings with imperial dimensions, if not then taking measurements from something made to round numbers of inches or feet using the same units is much more practical, but when they come to make the model they will be using metric machine tools, measuring instruments and metric dimensioned stock material.
Believe me it was the first time I’d ever heard that friend or any of his model railway and real railway society friends mention any metric measurement in any context. They were firmly stuck in the past. In my experience scale is more usually expressed as a ratio, 1:x. I’m surprised they use 4mm:foot not 1mm:3” or even better 1:176.
Well as PCPA says there are advantages to this form of nomenclature - even though I doubt his thoughts about how it came about. The whole history of model railway scales is complex with some clearly based on imperial and others with a much less clear lineage.
So if building a locomotive with 6ft driving wheels, I know I have to buy 24mm wheels without recourse to the calculator. A 10ft wheelbase wagon has wheels spaced at 40mm.
Historically we are building things that were built to imperial measures but doing the building in metric so that odd scale convention can be made to work for you. As I wrote above however, I di fully accept that it is an odd way to express scale.
I agree, regarding using a scale like 1:176 which I think was expressed, ISO preferred standard scales have been in existence for at least 50 years, they were around well before I was trained as a draughtsman, whilst it might suit me to use a scale like that because I have been given measurements in feet and at that scale the view can be drawn on say an A4 sheet using a mm ruler it should be avoided and the standard scales (SI scale ratios using 2, 5, 10 and 20)used.
The main reason for this is that the person reading the drawing can use a scale rule which are made using the standard ratios, there are also Imperial scale rules of inches and fractions of inches per foot.
Of course any drawing blank will be printed with "do not scale drawing!"
Longer, it depends on shrinkage ration of the metal being cast, I inherited my fathers carpentry and engineering tools including pattermakers rules, he taught me from a very early age never to use them except for their intended purpose.