Hi Michael and Ralf,

Michael - many thanks for taking a look at the .ini option for specifying locations of external scripts.

Ralf, You're a star. Thank you :)
Modifying NACAAirfoilGenerator.js as you suggested cured the problem in MoI v4 - on both PC and MAC.
Out of interest, how did you debug it? Or did you find/remember some change of method to accessing ObjectList items in v4-compatible scripts?

I've become quite rusty with scripting since I wrote it and nothing really stood out in the script documentation http://moi3d.com/wiki/Scripting especially Max's mirror of Dave Morrill's script documentation/api/

It'll be interesting to hear from Michael if this change of access was intentional :)