Editing Protofour (section)

{{Infobox model rail scale
| name = P4
| image_filename = 
| image_caption = 
| widthpx = 
| scale = 4&nbsp;mm to {{cvt|1|ft|mm|0}}
| ratio = 1:76.2
| gauge = {{RailGauge|18.83mm}}
| prototype_gauge = [[Standard gauge]]
| website = {{URL |https://www.scalefour.org/}}
}}

'''Protofour''' or '''P4''' is a set of standards for [[Rail transport modelling|model railways]] allowing construction of models to a scale of 4&nbsp;mm to {{convert|1|ft|mm|abbr=on|order=flip}} (1:76.2),<ref name=p4intro /> the predominant scale of model railways of the British prototype. For historical reasons almost all manufacturers of British prototype models use [[OO gauge]] (1:76.2 models running on {{RailGauge|16.5mm}} gauge track). There are several [[finescale standard]]s which have been developed to enable more accurate models than OO, and P4 is the most accurate in common use.

The P4 standards specify a scale model track gauge of {{RailGauge|18.83mm}} 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.<ref name=clagtimeline /> 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 [[Train wheel|wheel profile]] and track parameters to use, which are largely a scaled-down version of real-life standards with some allowances for practical [[Engineering tolerance|manufacturing tolerances]].

P4 standards have been extended to several other prototypes. Broader than standard gauges have been modelled using P4 standards, including [[Brunel gauge|Brunel's]] {{RailGauge|7ft0.25in}} gauge, modelled with {{RailGauge|28.08mm}} track<ref name=p4standard />{{rp|2}}<ref>{{cite web |title=BGS Standards |url=http://www.broadgauge.org.uk/modelling/models_standards.html |publisher=Broad Gauge Society |access-date=2021-05-11}}</ref> and Irish P4, the {{RailGauge|5ft3in}} [[Irish gauge|Irish broad gauge]] modelled in P4 in 4&nbsp;mm scale with {{RailGauge|21mm}} gauge track.<ref name=p4standard />{{rp|2}}<ref>[http://website.lineone.net/%7Esjohnson40/ Steve Johnson's Irish P4 Models]</ref> 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<ref name=p4standard /> and the society's Central London Area Group (CLAG) make a HTML version available.<ref name=p4html />