All my textures are 4K by default, but I do supply separate downloads of resized textures at 2K and smaller if people wish to use those instead. Some of my models have 100 4K textures spread over colour, normal and alpha maps. 4K is probably excessive for a game engine, but doesn't seem to cause issues elsewhere so I would say 4K is the way to go, but perhaps with downsized options. Only a small percentage of customer download my 2K texture variations and virtually no one downloads the smaller ones.