I am not a fan of the C4 on the mastering bus. I think it is one of those things that can sound good at first, and then you go back to it and say "why did I do that?".
If you are having troublegetting loudness, you need to try putting your mix in a channel and making a buss with a softclipper and a limiter on it. Turn up the channel fader to the top and send it to the bus channel. turn up the softclipper 1 or 2 db and set your limiter to take off 1 or 2 db max.
This way you have achieved a lot of loudness with almost no compression.