Only the Jon and Will know for sure when (or if) Sublime version 4 will be launched, so anything we say in this thread is pure speculation unless someone can cite a credible source. I myself have only really followed Sublime since early April of this year, so I'm by no means an expert or long-standing user.
With that said, from the Sublime Text Blog, this timeline is available:
So, if history holds (and again we have no way of knowing if it will or not) even if version 3 was released tomorrow it could still be months or a year before a beta version of 4 is available, and even then it could be available only for registered users.
The Sales FAQ does mention that "Future major versions, such as Sublime Text 4, will be a paid upgrade" and "Licenses purchased for Sublime Text 3 do not expire, however an upgrade fee will be required for Sublime Text 4" (emphasis mine).
So we can probably safely presume that we will have to pay something less than a full purchase price for the upgrade when it happens, since that's usually how that works. Anecdotal evidence in the post that announced the first beta version for Sublime 3.0 appears to be that the upgrade price is less than half the price of a full new version.
My own advice would be that if you find Sublime Text 3 useful to you to the point that you're actively thinking of buying a license, I wouldn't let the potential possible release of an unannounced new version slow you down. It could happen tomorrow or it could happen months from now, and that's months of time that you could have been happily using your license.
I realize that this advice isn't for everyone, though, especially if the cost of the license is hard to swing. I hate posts like this that gloss over how not everyone can afford the tools that they want to.
All I can say is that for myself personally, I used the trial version of Sublime 3 for less than a week before buying a license and in the intervening months I have not regretted that decision once. If version 4 was available tomorrow and there was no upgrade and I had to pay full cost, I would gladly do so (after checking out the new version first, of course ).