The number of "filters", as I understand it, has to do with the # of filters that the computerized signal information has to go through, as the detector determines what info is minerals to account for. So the more filters it has, the better able to "filter" out mineralization from the signal/soil.
4 filter machines would be ones like the XLT, 6000 di pro, etc.... They are usually faster sweep machines requiring a bit more motion to get depth. But not as bad as yester-years 4 filter machines which had to be swept REAL fast
Good places of these machines would be the wet salt beach, deeper targets in turf, faster coverage through furroughed fields, etc...
2 filter machines have better target separation, and better averagaing (see-through) ability. But they generally don't get as much depth as the 4 filter machines, and don't work as well in nasty minerals. Good places for these would be ruins/relicky sites, where nails and iron abounds.
I keep one of each and pull them out depending on what sites I'm going to hit.