1 of your personal referees will need to provide gap references for any gaps in employment (verifying who you are and what you were doing), in addition to character references. They will be contacted twice (2nd time by the Airports Authourity) to verify information. I am guessing the other 3 will give character references (who you are as a person). As to why 4 personal referees who have known you for 5 years sociably, when 2 can suffice, I have no idea!!