There will be at least 3 more betas before the GM version (based on previous years),
We don't know how many there will be but they will be focused on fixing current issues at this point, not introducing new ones.
Features have to be tested, large and small, and not just in house. Blurred layers are extremely taxing. In any case however, by all the evidence, Apple has already tested and deemed blurred layers should not be supported on anything older than a 4S.
Developers have to know how their apps will react on older phones and what resources are available, as well.
The iPhone 4 isn't that old, as a matter of fact the iPhone 3GS had versions 3.0, 4.0, 5.0 and 6.0.
Actually, with the impending release of a "cheaper" iPhone now, it's a whole different ballgame and Apple is not one to worry about leaving out of date devices, they will no longer sell, behind when it comes to newer features.
The 3GS was not able to run every feature introduced in later releases, just as the 4 will not be able to do so now (which was already the case with iOS 6).