I do agree it'd be market saturation, especially for the likes of the Bandon Tank and the J26, which have not only brass kits in 4mm but also 3d printed consideration in the near future.
That said, what seems to be most popular so far is
J18 (4/5)
Elf (3)
J30 (2/3)
Only the J30 has any form of model in 4mm, that being a 3d print.
Somewhat surprised the Ivatt F6 hasn't received any love!