Keep in mind that 4% raises your shield mitigation from 81% to 85%.  This is effectively a 21% damage reduction under ideal conditions, since you're blocking what little damage does get through.  Shield mitigation upgrades are one of the few types which actually give accumulated returns, rather than diminishing returns.

correct me if im wrong but if you're mitigating 85% of damage that should mean you're only taking 15% of the incoming damage...i think this is applied AFTER damage modifiers between weapons/armor types though. 

If you want to see what 21% increase in survivability looks like...

A ship with 5000 combined hull and shield taking a constant 100 DPS at 81% mitigation will take 19 DPS (ignoring other gameplay mechanics) for a lifetime of 263.2 seconds. Raise the mitigation to 85% and the ship's lifetime increases to 333.3 seconds, a difference of 70 seconds.

You can find these values by providing values for the formula Lifetime = HP/(DPS*(1-Mitigation))

That formula has these implications (some are rather obvious):
1. Total hull and shield (HP) is proportional to lifetime (that is, as HP increases, lifetime increases)
2. Total DPS is inversely proportional to lifetime (as DPS increases, lifetime decreases)
3. Mitigation is proportional to lifetime
4. Mitigation increases at higher percentages have more effect than the same increase at lower percentages (an increase from 15% to 20% has less of an effect than 45% to 50%)