The four moveset limit is what keeps many Pokemon viable. With five moves, only the most powerful pokes with the best coverage would stay - pokemon like Salamence, Hydreigon, Dragonite, Tyranitar, Metagross, and so forth. Lots of lesser mons would no longer be able to compete because pokemon have an extra move and gain even more SE coverage.
It would be great. 6th generation in and we still have 4 moves? It's like Call of Duty, same game every year.
Had to get "that one guy" in early.
Now rest of the topic will be okay.
On topic: I often thought about what it'd be like with 5 moves. One thing I thought about is that Mega Evolutions have a signature move that gets planted in the middle of the four moves on the battle screen, while the four moves they know already make like "L" shapes around it.
So they'd have a signature move, and the 4 they already know. Yeah, Mega's are already powerful enough, but I thought it'd be a fun thing.
Official President of the NST 'But Neku, I thought you couldn't afford to lose? Give up on yourself, and you give up on the world.' - Joshua