Most textures are 4k. The entrance building and the backwall segment have 8k, the terrain might get 8 or 16k. Hope thats enough
4k minimum should help, yes. And I'm all for the higher rez ones!
I do all my renders exporting to LuxRender, and it tends to be pretty brutal to low-rez textures. I often have to replace environment prop textures, because they've crammed way too many surfaces into a single UV map and then shipped a 2k image for that one map...
(LuxRender is also why I prefer normal maps over bump maps; normals give much more consistent/predictable behavior than bumps do in LuxRender.)