P4 or Protofour is a set of track standards using 4mm:1ft scale - the same as OO gauge - but with a slightly wider and more accurate track gauge. P4 modelling standards are supported by the Scalefour Society and the EM Gauge Society.
The most popular scale for railway modelling in the UK is 4mm:1ft. That means for every 1ft measured in real life, in miniature, it'll measure 4mm. The majority of modellers choose to practise the scale in OO gauge, with a track gauge (the separation between the two rails) of 16.5mm. For some, this compromise of track with over-sized bodyshells simply won't suffice and they seek better scale accuracy where track and wheel standards are concerned. P4 was developped to widen the track gauge from 16.5mm to a proportionally closer 18.83mm. The standards were later published in Model Railway News by the Model Railway Study Group in August, 1966.
The widened track must be hand-built using components available from a variety of suppliers and modellers can either build locomotives and stock to suit the wider track, or increasingly, adapt RTR locomotives and rolling stock. Wheel profiles change from OO gauge standards, too, using finer flanges and check gauges.
'Bodmin' in P4 featured in the April 2020 issue of BRM and BRM TV. The layout was built in the 1980s to P4 modelling standards.
The specifications of P4 and its modelling is supported and promoted by the Scalefour Society which publishes a journal called Scalefour News and whose membership organises exhibitions around the country. Modelling in P4 requires a greater degree of accuracy when setting up a locomotive because smaller track imperfections can be liable for causing a locomotive to derail. For this reason, many locomotives in P4 have a degree of compensation - or springing - to their axles, ensuring contact between wheel tread and rail head.
P4 has been a more popular choice than HO, for those looking to better the scale proportions of their OO gauge models, mostly because it's easier to make the wheels and track wider than the locomotive and rolling stock bodies shorter. BRM magazine features a handful of P4 layouts every year and the construction of layouts to these standards remains a popular niche of the hobby.