Hi Ulisses, most v3 scripts can be used directly in v4. There were a small number that needed minor updates. The V4 script parser is stricter about syntax errors and will throw an error on invalid code while the script parser in v3 would silently ignore some of those things.

V3 customized UI files cannot be directly used on v4 but they can be adapted. Here's a recent one:
http://moi3d.com/forum/index.php?webtag=MOI&msg=10095.9

- Michael