The biggest difference is that Pr CS6 is a 64bit application,
and requires a 64bit operating system.
CS4 is a 32bit application.
CS4 can likely import your DSLR media with the addition of
MainConcept's MPEG Pro HD plugin, but performance will be
nowhere near CS6 capability.
Be aware there is an outstanding AVCHD bug in CS6,
but the release of a fix in the undetermined future has been promised.
Audio and Video glitches | AVCHD footage - CS6 bug
Premiere Pro CC July 2013 Update
"Our commitment to customers includes those who haven't yet
subscribed to Creative Cloud or who rely on CS6, and we plan to
release an update to Premiere Pro CS6 in the coming weeks that
will address an AVCHD clip spanning issue."
CS6 Trial Download - where can I find it?
CS6 - Buy the Perpetual License version (Not Creative Cloud rental)
To buy an upgrade instead the of full version, click the buy button
then select 'I want to buy: 'Upgrade' instead of 'Full'.
CS5 and CS5.5 Products are eligible for upgrade to CS6.
Thank you, Joe!