Master Critical just means that the 4 weapons you have weapon focus in (this is a feat requirement to class as a man at arms) will have the Improved Critical feat.

:: Improved Critical ::
"Type of Feat: General
Prerequisite: Proficiency with the chosen weapon, base attack bonus +8 or higher.
Specifics: Combat ability doubles the critical threat range with a given weapon. A longsword that normally threatens a critical on a roll of 19-20 would now threaten a critical on a roll of 17-20.
Use: Automatic. The threat range of a Keen weapon is already doubled, increasing to triple with this feat. This feat can be selected multiple times, applying to a new weapon category each time.
Special: Halflings and Gnomes are small creatures and as such they can never use the following large weapons - dire mace, double bladed sword, doubleaxe, greataxe, greatsword, halberd, heavy flail, and scythe."

If you are given 4 new feat picks when you reach level 3 of Man at Arms something is probably wrong : D

Also I think choosing improved critical for weapons that you have weapon focus feat in don't stack with the bonus from master critical so it may be redundant.