Protofour

Protofour or P4 is a set of standards for model railways allowing construction of models to a scale of 4 mm to 1 ft (1:76.2), the predominant scale of model railways of the British prototype. For historical reasons almost all manufacturers of British prototype models use 00 gauge (1:76.2 models running on  gauge track). There are several finescale standards which have been developed to enable more accurate models than 00, and P4 is the most accurate in common use.

The P4 standards specify a scale model track gauge of for standard gauge railways. Joe Brook Smith was the first to propose use of an exact scale track gauge in July 1964, when also the term “Protofour” was invented by Malcolm Cross. The standards were later published in Model Railway News by the Model Railway Study Group in August 1966.

Just as in the prototype railway, on a model the wheel-rail interface is the fundamental aspect of reliable operation. So as well as a track gauge, P4 also specifies the wheel profile and track parameters to use, which are largely a scaled-down version of real-life standards with some allowances for practical manufacturing tolerances.

P4 standards have been extended to several other prototypes. Broader than standard gauges have been modelled using P4 standards, including Brunel's gauge, modelled with  track  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 and the society's Central London Area Group (CLAG) make a HTML version available.

S4 Standard
The S4 Standard is maintained as part of the P4 standards. The S4 Standard removes an allowance in the P4 standards for the tight (compared to the prototype) curves used on model railways. The wheel back to back in S4 is therefore slightly wider than P4, and the related track work dimensions are dead-scale.