Hi Lewis, the CustomUI mod from Max is different for v4. There is a new version of it for v4 and it is installed differently, in v4 it goes in the MoI appdata startup folder.

You can get it at https://moi3d.com/forum/index.php?webtag=MOI&msg=6507.1 , use the link for the v4 one not the one for v3.

- Michael