Through trial and error I got Compressor 4 to access all cores on my 2009 8-core MacPro. The most important piece of info not in the help menu for people who have set up a QuickCluster with Compressor 3 is that you must open System Preferences and stop sharing in the old QMaster pref pane before you can set up a new QuickCluster thru the QMaster menu in Compressor 4. After that read the help menu for the rest of the instructions. I haven't tested the new QMaster to see if I can increase the number of Compressor instances to 4 from 3 as I have them now without having a processor collision failure.

Also, many of the presets from Compressor 3 are missing here, such as DV-NTSC. It seems Apple wants everyone to go ProRes. However it's easy enough to create new presets. However I did a test, to make a 9 minute DV file from a SD h.264 took 20 minutes. To make a ProRes file took 2 minutes and 40 secs with the file sizes nearly the same. Why the huge time difference?

While doing the tests did you watch actual CPU utilization through activity monitor? My guess is that only some codecs have been optimized/enabled at this point to make full use of all cores on a machine, or that certain ones are accelerated by the GPU.

Yes I did watch with activity monitor and it was very curious to say the least. The DV codec set half the processors going to 75%, the other half around 20%. The ProRes codec barely registered!

Can the new compressor replace the old version? Will FCP7 use it or will I need to keep both versions of compressor on my machine till FCPX matures?

That's a good question. I see no need for the old Compressor yet, the new one is identical in appearance. the Batch Monitor is gone, replaced by the new Share Monitor. Perhaps if someone needs the old presets, the old version will come in handy but I not sure how that will affect the new implementation of QMaster.