Hey Barney,
C4 - always a great investment. If thats your goal, Id suggest you save up for the second lot. You will always find other things to spend certs on, so it could take a while if you dont save
RL - Decimator is good for close up anti-Max work but the phoenix is very versatile. Id suggest you would kill more with it.
AV Nade - cant comment - dont know.
Second grip upgrade - cant comment - dont know.
Im sure someone else will jump in and help you out a bit more.... see you out there