The ID of the hose will not make a blind bit of difference to anything.
...........

In the same way the piston size in a brake caliper is irrelevant......![]()
I didn't see anyone mention brake calipers or pistons?? In the context of the post then the vac line ID is irrelavent. Changing from a 3mm id pipe to 4mm pipe isn't going to make any difference at all.
Next you'll be heatsheilding the vac lines![]()