Its not so difficult to stretch the 4mm ID open with some needle nose pliers. With that said we now have awesome "DuoTight" push in fittings which eliminate this issue all tgoether.
So i am assuming you are using ball lock kegs so if that is the case you should just get this fitting:
https://www.kegland.com.au/duotight-8mm-x-ffl-to-fit-mfl-disconnects-7-16-20unf-1-4-sae.html
(you can also use this on your regualtor to)
Or of you are using commercial keg couplers then this is the fitting you need:
https://www.kegland.com.au/duotight-8mm-push-in-to-5-8-to-suit-keg-couplers-and-tap-shanks.html
then the only join left is the one at the back of the tap shank. Due to the tight space in the tap shank we have not got a push in fitting that works in this spot yet but we have got a solution for this soon. So in the very near future we will start providing short shanks with a special push in fitting at the back of the shank for this application.
These fittings appear they need a thread instead of a barb, so would require new disconnects and an appropriate fitting for the reg?