Ultimate_4k_textures_pt4(install).zip -

A list of artists or original sources for the 4K assets.

Instructions telling the mod manager where to move specific .dds (texture) files. ULTIMATE_4K_Textures_PT4(Install).zip

If the "(Install)" tag refers to a installer (common in Nexus Mods), the most important text file is the XML script that governs the installation menu. It contains: A list of artists or original sources for the 4K assets

Details on what was updated in "Part 4" (PT4) specifically. ULTIMATE_4K_Textures_PT4(Install).zip