If you are connecting the 6MM from the UAT to the pump, just put a small piece ( 1/4" or 1 cm, just the length of the nipple) of the smaller 4mm line over the fuel pump inlet nipple. Then just slide the 6mm tubing over the 4mm tubing you just placed on the nipple. Secure with wire tie but not necessary.
Been fine for several installations. Festos should not be used on the suction side of the pump.
BC