You don't need a reducer!

Put a short length of 4 mm. line on the pump inlet nipple. It should be no longer than the nipple. Then slide the 6 mm. line over that. It provides a nice snug fit. Safety wire it just like all of your fuel fittings.

I've never had a problem with this set-up in hundreds of flights. Frankly I thought that's the way everyone did it, since that is what was in the Ram instruction manual.

Joe