Protofour or P4 is a set of standards for model railways allowing construction of models to a scale of 4 mm to (1:76.2), and Irish P4, the Irish broad gauge modelled in P4 in 4 mm scale with gauge track. Several successful models of narrow gauge prototypes with a correspondingly accurate track gauges have also been produced to P4 standards.
P4 standards are promoted worldwide by the Scalefour Society, which is based in the United Kingdom. The EM Gauge Society also provides support for modelling to P4 standards: many P4 modellers belong to both societies. The standards document is hosted by the Scalefour Society
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