There is no reason or sense in reducing the available fuel to the pump by aa reduction of the supply line cross section.
So why are the solenoids designed for 4mm hose?
The 6mm UAT to 4mm solenoid fitting is, IMHO, a complete pain in the ass. Why can't BVM make the UAT with a 4mm nipple. Or, if the fuel line cross section is that important, why can't Jetcat (and others) make the solenoids with 6mm fittings?
Considering the price of the UAT, it seems a pretty stupid design decision.