Well, I assume from your description that there are four LNB's as well, fed, I assume, off their respective receivers.
So that would eliminate that problem--unless you do have a common supply OR some kind of cross-fed situation using a DC blocker which suddenly decided to let DC through...shorting out or dragging down supplies because they are now coupled?
Dish alignment is always a problem, though four dishes at once? --if you've had a wind storm. I have heard of a WV-type problem, where the dish is mounted on a hillside (in a blob of concrete no less) and slowly lost alignment as the hillside slid downhill..
Hence the likely culprit is some kind of TI--or even airborne interference.
Military radar is a sneaky culprit. Depending upon plans unknown to us common folk, it can suddenly appear for a week, then vanish. Bush visiting your area, perchance? Usally it doesn't kill the signal like you see but instead causes sporadic dropouts that get worse than get better.
Military Radar is supposedly below the band used for satellite, but that assumes it is is where it is supposed to be transmitting. Microwave Filter can sell you a filter that hangs on your LNB & is very good at eliminating this problem. Somwhat pricey at $500.
If you have a friendly cable guy or even versatile two way shop you may find someone with a spectrum analyser. Our local two way guy has one that plugs right into the 1.2 ghz output of the LNB. His will power the LNB, so if you have a spare one, you can wave it around to see where any TI is coming from.
One type of TI he's found is from cell phone equipment on the fritts--but this is likely to be a pulse that sails up and down through the band.
Same thing can happen with STL's.