Unfortunately I don't think the SR3 texture utilities work properly for SR4. The .cpeg files can be unpacked and repacked, however in-game the textures are missing/don't work.
I am fairly certain I am repacking the .str2_pc (with the appropriate .asm_pc file) correctly, so I am pretty sure the SR3 texture utility is not compatible, unless I am doing something drastically wrong.
Even though I've never used the SR3 texture tool to convert my textures, I think it should work fine since texture format between the two games are the same.
But when I used ThomasJepp.SaintsRow.Stream2.exe to update my ASM file, the game stuck in the eternal loading screen, like the ASM is incorrect.
So I tried updating the ASM with the packer's /asm command, which as I understand only update the files being packed instead of the whole folder, the game with my new textures work fine.
So give that a try.