Glad to know your Pi's are up and running with 4 feeders.
Hopefully filter will help, but this varies from location to location and setup to setup. You will only know after trying it.
Meanwhile another thing to play with is dongle's gain which can be set to values between 0 and 49.6. Two more settings used are "max" which sets gain value to 49.6, and -10 which switches dongle to AGC (automatic gain control) mode.
(1) The signal from Mobile/Pager is strong, reducing gain some times help to overcome their deafning effect.
(2) The signal from nearby planes is strong. For example if you live close to an airport, you will notice planes disappear from your map when these are very close to airport. When these are close, the signal is stronger than the upper threshold of the dongle, and rejected as garbage. Reducing gain in this case often helps.
(3) Reducing gain to see nearby planes is not without a cost. The signals from far away planes near the maximum limit (say 200nm to 350 nm) are very weak, and reducing gain too much can result in these going below the lower threshold of dongle, and not detected/processed. You will loose these far away planes from map, and your maximum range will reduce.
Overall it is a compromise situation. You can try different gain values such as 50, 45, 40, 35, 30, 25 and at each setting leave the system running for a while and compare max range & plane count to decide which gain value suits you best.