I use a multiband G5RV from WIMO.de with 10m or so of twinlead copper ended
with a balun 4:1 and coax.

If I were you, I would remove this 4:1 balun and solder the coax
direct to the twinlead. I would then use good coax right to the
radio.

The reason is: the impedance at the solder point is not 300 ohms and
with your balun, you're inserting more losses .

73, de Stef