I agree starting de-deprecating the core is great step in the right direction.
However, I believe doing this in 1.6.4 is a HUGE mistake. Let me explain why. Although anyone can version the software in any way there are certain mini-standards to follow. EG: If using versions with 3 numeric positions, the digit on the right is usually incresed when there have been done only bug fixes, the digit on the middle involve minor features added and the digit in the left implies major features or changes.
I believe most people will assume chaning from 1.6.3 to 1.6.4 will only result in a couple of bugs fixed but that the upgrade will still be compatible with their current version. However, this will not be the case. Anyone hosting the application on a hosting server using PHP 4, or even PHP 5 earlier than the 5.1.6 will find their sites not working after the upgrade.
I believe the best approach would be to create a 1.7-dev branch and also a 1.6.4-dev branch (to add the latest very minor changes). Then, whenever 1.7 is ready, release this PHP 4 support drop (making sure every piece of documentation is updated... particularly the q2a home page that states it runs in PHP 4).
As a side note, I believe there should be a clearer understanding on what each number in the version means. That would help people understand the impact of upgrading.
Just my 2 cents.