Looking good!
As 6pt said, SJS have stepped ferrules http://www.sjscycles.co.uk/stepped-chrome-ferrules-37-mm-insert-for-5-mm-gear-outer-pack-of-4-prod5267/
they worked for me but they're made for 5mm outers, so a bit loose. let me know if you find a place selling them for 4mm outers.
I had the same issue, ended out using the plastic casing from a 5mm brake cable as a shim. Nice and snug now.
Rad bike also.
@ptr started
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Looking good!
As 6pt said, SJS have stepped ferrules
http://www.sjscycles.co.uk/stepped-chrome-ferrules-37-mm-insert-for-5-mm-gear-outer-pack-of-4-prod5267/
they worked for me but they're made for 5mm outers, so a bit loose. let me know if you find a place selling them for 4mm outers.