Normally they propose a 3 à 4 times the last bet. So a 9bb bet would result in something between 27 and 36bb's
4b do not need to be this large unless your just value betting a complete fish you know is going to call/GII light.
I use a 2.2x for 4b (roughly) so my sizing in the above example would generally be 20bb ish. This not only makes it look like you could still lay your hand down to a shove, inducing bluffs occasionally, but gives you a good price on your own 4b bluffs (sporadic at micros), when called your still playing for stacks and if they were going to fold to your larger 4b they are still likely to fold to this sizing, the times they don't they will shove lighter and call lighter giving you a greater
equity advantage against there range.
This is for cash games I should add.