I personally cannot get on with acrylics so tend to stick with enamels for most things. I do use the cheap tubes of acrylics from the likes of The Works though for scenic work just to provide base level of colour to things.
I think I have generally used either a B or even top E before.
Alan.
Edit to say bass guitar strings can be useful for vacuum pipes and the likes as well.
Google search finds RM July 1989 article by Peter Denny for building a model gasworks but the cover doesn't look familiar to me so I'm not sure if this is the one I was thinking about or not.
Chaz,
Hope you don't mind the incursion, but this motorcycle kit (complete with spokes, although probably overscale) was from Blackberry Way. Unfortunately they do not appear to trade any longer.
Evergreen make some half round section plastic strips which is good for guttering. Although its solid, not hollow, it is not really noticeable. I model in 7mm scale and use it but I am sure they do something smaller suitable for 4mm scale.
Alan.