> Is 4mm bungee about the optimum diameter for adding some 'self tensioning' stretch to a guyline?
Well, I think Eric's comment about over-engineering might be appropriate here!
It does depend on what sort of tent you have and where you put the bungee cord. I use it on the down-wind end (vestibule) of my tunnel tents, and it is a loop about 6" long of 4 mm bungee at each corner. Yeah, strong. On the windward end I use loops of 3 mm nylon cord: no movement allowed at that end!
But while some lengthwise tension is a good thing on a tunnel, it may not be so good on some other tents or some other places. For instance, I would not use bungee on the side guys on my tunnels. Rather, they get set to the right length for the pole arches and stay that way for the night. Any slackness in the fabric due to water or cold gets taken out lengthwise, by the bungee cord.