While you state they are getting 4 times the messages how do you compare in terms of aircraft seen on the map at the same time, your post seems to indicate they are seeing the same number of aircraft as yourself messages does not always equal more aircraft picked up
Where are you located and what is your station number to allow a look at your location to see if there may be topography issues that may impact reception for example are you in a valley compared to the ones you are comparing yourself to, are there hills, trees, buildings etc that may be shielding your antenna reception compared to the comparison stations you are using. What type of antenna do you use
What type of messages make up the various totals and how do yours compare, ADSB with positions, MLAT, UAT etc is there one message type that is making the difference
Alan