I don't follow the four move rule. In my writing, I prefer to think of pokemon learning virtually any move within the canon's constraints, but being limited by their intelligence (intelligent pokemon will be able to learn new moves easier and be able to use the moves they know creatively), breeding issues with in real life would be impossible (skitty and wailord, for example) and obviously the wealth of the owners, which determine what moves can be 'bought and taught' as it were.
In real life, I think that a single move would be far more varied; in strength, duration, execution and so on, mostly due to the pokemon using that attack. Different pokemon species are going to use the same attacks in a slightly different way, older, more experienced pokemon are going to use an attack differently from how a younger, inexperiened pokemon would.