What's so bad about a mon having the '4MSS'. I understand that a mon can need a specific move to beat something but why is the element of surprise ( example: most lati's don't carry t-bolt and can surprise gyara) never considered as good?
4MSS is commonly misused in that sense. The version you described is for mons like Infernape, the Latis, Hoopa, or Keldeo, who deal with some checks their "main" moves don't cover and then leave remaining ones to their teammates.
Legitimate 4MSS refers to when a mon cannot function optimally in its ideal role with only 4 moves.
Banette for example wants Priority, Status, Destiny Bond, Taunt, and a decent Attacking move, but must give one of them up and loses functionality as an overall utility mon because of it.
- Without Priority, Banette is much more vulnerable to revenge killing because of its low speed and bulk
- Destiny Bond is a big niche for discouraging Revenge Kills or grabbing momentum against offense, a team where momentum matters a LOT
- It needs both Taunt and Status to neuter opponents and Stallbreak, or it becomes deadweight against bulkier teams.
- Without an attacking move, it becomes too passive since its priority moves are not strong enough to beat most opponents from full.
The Pokemon I listed before would be fully functional as Wallbreakers/Sweepers without the moves fighting over their last slot for "4MSS", whereas Banette's case is a Pokemon that has trouble in its basic role, much less in carving a niche in said role against its competition, because it cannot cover everything it needs in four moves.