What size cable combs? Over size or under size?

[Dominic-Toretto]

I am looking to buy some cable combs, but am not sure which size to get.

My silverstone cables are slightly over 3mm, should I undersize and get the 3mm cable comb, or try and use the 4mm?

Additionally asside from the cable combs them selves, is there anything else I will need?

Thanks all