P4 is not about the 18.83 gauge. Side on from the track, I find it hard to spot the differences between plain hand built track in 16.5mm, 18.2mm and 18.83mm. P4, for me, is about the other dimensions in the standard.
Firstly, there’s a nicely defined wheel profile that doesn’t look like you’re building a steamroller. This profile also helps when squeezing all the parts in a chassis together so it looks like a scaled down version of the real thing.
Secondly, the variously named check rails found in crossings are close to the scale distances away from the rails they lie next to. This makes the track-work look like a scale model, rather than a toy. Once the other details possible with hand built track are used, it can look almost like the real thing.
I think it’s a shame people often label layouts built to P4 standards as “18.83” layouts – to me it misses the point.