Sic,
These fuel pumps push a lot better than they pull. It is a good idea to have everything on the suction side 6 mm. to minimize flow restriction and make life easier for the pump.

Typically the UAT goes directly to the pump, which is why BVM made that line 6 mm. The solenoids are on the pressure side of the pump, which is why they have 4 mm. fittings. The design flaw, as I see it, is the pump manufacturers not having a 6 mm. fitting on the pump suction side.

Joe