If you are about to do lvl 4 missions, look up some fits for your ship, and then over tank it. Lvl 4 missions are drastacly harder than lvl 3s.
Go to this site:
http://eve-survival.org/wikka.php?wakka=missionreports and build your tank around your mission. The best bit of advice I can give you, is sheild tank, and boost your resistances as high as you can go. Dont worry about cap stable, as the setups usually arent as good as they could be.
Dont listen to what anyone says when they tell you that you are over tanking your lvl 4s, if you are first getting into them, other wise you WILL lose your ship.
I lost 3 Ravens that were supposedly "perfect" for lvl 4s, cap stable shield tanks that could "easily" tank lvl 4s (and a damn good Rokh, but that was due to my own stupidity:P). They were all crap fits. I then fit this:
[Raven, Raven]
Ballistic Control System II
Ballistic Control System II
Ballistic Control System II
Power Diagnostic System II
Power Diagnostic System II
X-Large Shield Booster II
Heat Dissipation Field II
Ballistic Deflection Field II
Heat Dissipation Field II
Shield Boost Amplifier I
Caldari Navy Shield Boost Amplifier
'Arbalest' Cruise Launcher I, Caldari Navy Paradise Cruise Missile
'Arbalest' Cruise Launcher I, Caldari Navy Paradise Cruise Missile
'Arbalest' Cruise Launcher I, Caldari Navy Paradise Cruise Missile
'Arbalest' Cruise Launcher I, Caldari Navy Paradise Cruise Missile
'Arbalest' Cruise Launcher I, Caldari Navy Paradise Cruise Missile
'Arbalest' Cruise Launcher I, Caldari Navy Paradise Cruise Missile
[empty high slot]
[empty high slot]
Large Anti-EM Screen Reinforcer I
Large Anti-Thermal Screen Reinforcer I
Large Anti-Kinetic Screen Reinforcer I
Hobgoblin II x5
Hammerhead II x5
Yes, it is over tanked, and by a large margin, but I didnt lose it, and was plowing through lvl 4 missions quite quickly, but more importantly safely till I got used to it. Eventually, after I got used to how it tanked (pulsing the shield booster only when needed as it only has 1:30 of cap, which is plenty btw) I started dropping tank mods for more damage. I dropped the EM rig for a Rigor, and dropped the 2 amps for 2 tps, and dropped one PDU for another BCS.
Moral of the story/ TL;DR: Over tank till you are comfortable and used to 4s so you dont lose your ship.
Also, T2 Drones are a MUST for popping scramming frigates