Another possible reason for moving from 5 to 4 for a fireteam is the standard fireteam has 4 members (at least in the USMC) - a leader, automatic weapon gunner, and 2 rifflemen. It's a small unit capable of quick and independent maneuvers.
It also follows the "Rule of 3" - leaders can most effectively lead 3 "units" at a time. 3 members (and a leader) in a fireteam, 3 fireteams in a squad, 3 squads in a company, etc. This provides each leader 3 units to lead, and as leadership skills increase this scales up to the Division / Group levels.
I feel this works well well for CoD sets - I like the realistic sets, so why not realistic units?