If an Exit u/s a 157 seat aircraft may be reduced to 100 pax by the MEL, still 4 cabin crew reqired, Purser or Cabin Manager mandatory.
This is not necessarily an accurate reflection of the requirements for all CAAs. For example, the MEL relief may or may not be available for a single-aisle aeroplanes as there are CAAs that only permit relief for twin-aisle aeroplanes, not single-aisle. As well, some CAAs specify that the "Purser or Cabin Manager" is a member of the minimum cabin crew required not an additional crew member -- so in your example, just to clarify, it would still be four.
It should also be noted that there are some CAAs that permit a ratio of 1:50 passengers, although the CASA proposal is based on the number of configured seats.
There really is no "international standard" to speak of as ICAO hasn't specified one. Chapter 12 of Annex 6 to the Convention on International Civil Aviation relates to the requirements for Cabin Crew. Section 12.1 addresses the Standard for the assignment of emergency duties as follows:
"An operator shall establish, to the satisfaction of the State of the Operator, the minimum number of cabin crew required for each type of aeroplane, based on seating capacity or the number of passengers carried, in order to effect a safe and expeditious evacuation of the aeroplane, and the necessary functions to be performed in an emergency or a situation requiring emergency evacuation. The operator shall assign these functions for each type of aeroplane."