The P4 Standards (sometimes known as the “Protofour Standards”) were created by the Model Railway Study Group (MRSG), and are now maintaned by the society and its members as one of the Scalefour Digest series. Intended for use creating railway models at a scale of 4mm/ft (1:76.2), they are made available here for interested modellers, societies, and traders wishing to work and produce products to these standards.
Note that practical modelling to these standards is commonly done without a need to refer to this document - modellers simply use the jigs, etc available from the society and elsewhere that are marked as suitable for P4.
CLAG host a copy of the standard in web-friendly HTML.
Within the P4 Standards, the appendix “Use of 4mm scale equivalent dimensions (4ft 8½in gauge)” describes the S4 Standard (occasionally known as the “Scalefour Standard”). The P4 Standard is generally a scale representation of prototype dimensions, but it has an allowance made for wheels to have a slightly narrower back-to-back so that they can get around the tight curves found on model railways. The S4 Standard removes this, for an exact-scale set of dimensions.
The society generally promotes the use of the P4 standards, and the vast majority of its members model to P4 standards. The society documents the S4 standard for members with the space and skills to make suitable jigs to do so.