Based on some numbers I ran through a handy tool, if I have a short (40mm) length of 4mm tube in the run of 6mm, based on the flow rate of 500ml/min the pressure difference will be negligible.

2meters of 6mm pipe at 500ml/min results in 0.52 mBar - Reynolds number 1768.39 (Laminar Flow)

40mm of 4mm pipe at 500ml/min results in 0.53 mBar - Reynolds number 2652.58 (Transition region Flow)

so if I have a short length of thinner, flexible pipe for the valve, I can keep the same, proven design and not have any major pressure/flow issues. From what I understand from the Reynolds number, the smaller pipe is on the verge of turbulent flow eg causing chaotic eddies, vortices and other flow instabilities. Whereas the rest of the pipe will exhibit laminar flow eg smooth and, well straightforward.

I'm not sure if the internal diameters of the connectors, but they may end up by having more of an effect than the 4mm pipe sections... we shall see.