Yikes. I tried lvl 4s in a battlecruiser with around 250 DPS once and couldn’t finish it: The mission had battleships with a tank I literally could not break with DPS that low. Sooner or later you’ll run into that problem, too.
As my personal rule of thumb, you’d not even try a mission lvl. if you can’t reach this kind of DPS:
Lvl. 1 missions: below 100 DPS
Lvl. 2 missions: 100-200 DPS minimum
Lvl. 3 missions: 200-300 DPS minimum
Lvl. 4 missions. 400 DPS minimum
And this is really generous, since many lvl. 4 missions can be a pain to run if you are below 500-600 DPS. 400 is just the minimum necessary to even be able to complete most of them.
Otherwise, just do what everyone else has been suggesting, and you should have no problems.