Personal difficulty rating for Level 4 missions (as of 6/2008):
I just started doing L4's solo.

EASY
The Score
Duo of Death
Rogue Slave Trader 1/2
Massive Attack, Sansha
Recon 3/3
Infested Ruins


MED
Rogue Slave Trader 2/2
Guristas Extravaganza


HARD
Ritualist Raid
Silence the Informant


IMPOSSIBLE ATM
Pirate Invasion
Worlds Collide
Recon 1/3 (not blitzing)
Recon 2/3 (not blitzing or tricking)