I have one map showing this road. Between this point and highway 107 it is unnumbered. Between the point and highway 1001 it carries the number 1323. Same on Google maps. But they might be outdated and highway 1323 extended up to highway 107. May be it’s a good idea to ask the mapper who entered the number about the source.
In OSM the number is now 4046. I assume this is an error. National highways in the northwestern part of Thailand start with the digit 1. Highways starting with the digit 4 are in the south (Wikipedia). My maps and Google maps show 4046 in the south west of Trang.
I have travelled the same section myself about 2 years ago and recorded it as 1323.
I have found roads with numbers 4xxx all over Thailand, although they are certainly not national highways. I think we are talking about two different things here… I can only speculate, but perhaps the road has been demoted to local status, so the national govt. doesn’t need to pay for it any more.
From what you are saying, then, the section of road W of the marked junction doesn’t have a number, which checks with the ESRI GPS map. If you confirm this then the number needs to be removed and the road possibly be demoted to unclassified.
Incidentally, I have started to read “Captain Slash’s” reports and are downloading his tracks. Some are already in OSM. However, the next 2-3 months I will be more or less stuck at my in-laws’ place near Suphanburi, without phone or internet (bar mobile phone connection…), so I’m not sure whether I’ll be able to do much.