As others have said, you need a res-1 sensor to see the missiles.
Also, defending against 20-missile-salvos with point defense only is going to be damn hard. I´d suggest to start researching/building anti-missile missiles
I generaly put a twin meson or gauss turret on any warship (I am a big believer in "fleet defense") with a fleet of 6 to 8+ ships, the turrets add up a lot and provide decent point-defense, even against missiles much faster than their tracking speed (not enough against large salvos, but those are expected to be thinned out by my AMMs)
To counter the enemy ECM, there are two possiblilties
1. research Electronic Warfare --> ECCM
2. design a new Missile-Fire_Control with a larger range. Enemy ECM-3 will reduce your MFC-range by 30%. I usually put a MFC with 150 to 200% the max range of my missiles on (this also allows me to later on use more advanced missiles to their full capacity without a refit)
4 ships (one of which basicly a non-combatant) are not nearly enough to fight 20.000+t enemies (as you have noticed

Another thing I noticed, the ROF for your meson turrest is 10 seconds. This is odd, as with capacitor 3, 10cm mesons fire every 5 seconds. Are you using larger ones? If yes, scale them down. You gain in ROF and at the same time save some space, maybe enough to put some armor on the ships. If you use the turrets in final defense, the shorter range should not make much of a difference.
Oh, and you have one power plant too much. Yes, redundancy is nice, but (in my book at least) with small warships, and lets face it, a 5.000t warship isn´t exactely a dreadnought, once the internal hits start to, well, hit, it´s pretty much all over anyway. Better to put that mass into armor/shields and prevent those hits from damaging the internals in the first place.