Personal difficulty rating for Level 4 missions (as of 7/2008).
I started doing L4's solo in June '08. Using Raven and CNR.
The difficulty rating is rather a general aspect. For example: Silence the Informant or Stop the Thief are easy missions, if you get range first. If you don't and stay at the beacon (warp-in point) they are pretty hard to do.

EASY
The Score
Duo of Death
Rogue Slave Trader 1/2
Massive Attack
Recon 3/3
Angel Extravaganza
Guristas Extravaganza
Unauthorised Military Presence
Sansha Spies
Guristas Spies
Infested Ruins
Infiltrated Outposts

MED
Rogue Slave Trader 2/2
Rogue Drone Harassment
Ritualist Raids
Enemies Abound 1-4
The Right Hand of Zazzmatazz
The Assault
Gone Berserk
Attack of the Drones
Duo of Death

HARD
Silence the Informant
Stop the Thief
Intercept the Saboteurs
Smuggler Interception
The Blockade
Enemies Abound 5/5
Vengeance
Damsel in Distress
Cargo Delivery


VERY HARD (avoiding these due to the possibility of proximity aggro at warp-in)
Pirate Invasion
Worlds Collide

Setup nowadays is mostly like:
[high] 6-7 Cruise Launchers
[mid] XL-Booster, Cap Booster, Boost Amp, 3 hardeners
[low] 4 Ballistic Control Systems, 1 Damage Control
[rigs] Hardener Rigs on Raven, Cap Safeguards on CNR, Warhead Rigor Catalyst/Cap Control Circuit

I ditched the perma setup for a more active tank and damage.