"4k" textures are the textures that got added to the One X games for the purpose of those games being run at 4k that are not present in the One S versions. Install sizes on One X are substantially larger than install sizes on One S of games that have a 4k resolution mode because they're using larger textures. The exact texture resolution is irrelevant.What's "4K textures"? Do you mean the marketing term or the standard definition of a texture that is 4096*4096 pixels big? AFAIK those are pretty rare.
The XSS will be, at least generally, using lower resolution textures than the XSX because its resolution target is lower. The XSX's storage and ram configuration is based around the expected memory requirements for native 4k output, and the same is true of the XSS and 1440/1080p. The One X was not designed as a 1440/1080p box, it was designed as a 4k box. There shouldn't be any expectation that 4k specific optimizations would carry over to a console that isn't targeting 4k.