The reason why we do not have a Big 5 but rather Big 4 is not that Gretzky (or Orr, or Howe, or Lemieux) is so good that he lacks peers.
The reason there is no Big 5 is that there is no consensus #5 player all-time. People will argue for Bobby Hull, Beliveau, Hasek, Bourque, etc. in this position, and all them have an argument, and no one clearly has a much better argument than anyone else.
At the moment, Ovechkin is one of that group of players who have an argument for #5 all-time. Whoever your opponent picks as #5, it is always possible to say "Ovechkin is better or at least as good because..."
Ovechkin's legend is guaranteed to grow with time, just as Jagr's legend is growing. The longer Ovechkin spends without a heir who would in turn win 7-9 Rockets and challenge the career goals record, the greater is going to be the awe of future generations for what Ovechkin did.
It is possible, especially if Ovechkin adds to his resume and no one else emerges as a dominant force, that at some point of time Ovechkin will become a very common pick for #5 all-time. That would be the moment when Big 4 will turn into Big 5.
And no, it will not require an argument for Ovechkin>Lemieux or Ovechkin>Howe (even though I dread that future generations will make the latter argument - wrongly, but Howe does not get the respect he deserves even today).