4 pips in SYS is not a hard and fast rule - you don't necessarily have 4 pips in SYS all the time. If you are taking significant fire and you don't have 4 pips in SYS, though, you're making a mistake.
This is how I see it:
4 pips in SYS is for when you are taking fire.
4 pips in WEP is for when you are not taking fire, and you have your target in your sights.
4 pips in ENG is for when you are not taking fire, and you do not have your target in your sights.
I generally do not recommend 0 pips in SYS unless you are not being fired on at all (i.e. you are in your enemy's blind spot and can stay there, at which point 4 WEP 2 ENG is not a bad configuration. This is a lot harder to do since the AI upgrades, though!)
If you are dealing with a highly agile opponent and cannot get out of a disadvantaged position (i.e. you can't get him off your tail), you might need to use 3 ENG 3 SYS or 4 ENG 2 SYS briefly to break out of that sitaution, but don't leave yourself off of 4 SYS for long when taking fire.
And yes, if you've read my advocation of 4 ENG 2 SYS when fleeing in another thread, that's different from what I'm saying here - but in that scenario, I'm arguing a corner case in which I'm in a ship which is MUCH faster than my enemy's, and therefore won't be exposed to fire more than a couple of seconds. The only reason I'm making that argument there is because someone said that 4 SYS 2 ENG is always best when fleeing, so I intentionally concocted a scenario to demonstrate that the generalization is not in fact axiomatic (the generalizatoin is usually correct, however).