Hi,
The rate of fire of Milan III is 4.
To be honest, I ve never experienced a milan3 (with average -not even high- experience) firing 4 missile shots during reaction phase but,
I run a series of tests and almost always an ATGM (milan3) unit will fire 2 opportunity shots with the primary weapon(to be more accurate an average section of 3milan III will fire 5 reaction missile launches).
Then if the unit will get an additional reaction fire it will consist of small fire shots.
For me the most possible scenario was that 2 units of Milan3 were either together or close enough to give you the impression that it was the same unit.
Another possibility would be that the first 2 BMPs were destroyed by the primary weapon and the other two by point blank shots (range 1).
Anyway, don't really worry about this... probably you won't see this again in a generated battle.
cheers,
Pyros