With Sylveon only showcasing 4 attacks I think its still four. Besides that 5 attacks on Pokemon would upset the tiers for current meta as a lot of new formats would open up. it would probably throw the current meta into as much chaos as a new type would.
I do agree with Xander, changing the number of moves Pokemon can have at a time could upset the competitive balance and cause even more confusion, just like what would happen if a new type or triple-types were introduced.
There was talk before Ruby & Sapphire's release that the Pokemon would be able to have more than 4 moves at a time but that never happened.