Yes, both versions can co-exist peacefully on the same disk. Just be aware the DL only works within one version. So you can't dynamic link from PR CS5 to AE CS4 or the other way.
You will have to call Adobe sales... just other users here
But... from past discussions... I do NOT think Adobe will sell you the upgrade to just one part of the Master Collection... you must buy the matching product to receive the upgrade price
I have this configuration in my computer
I use Photoshop CS4, Media Encoder CS4, Premiere CS4, Encore CS4 full updated
Matrox RTX2 Updated
Windows 7 Ultimate 64 BIT Updated
Intel (R) Core (TM) i7 CPU firstname.lastname@example.org GHz
An ATI graphics card HD 4870 X2 Redeon,
With 12 GB of RAM.
This is something that you will want to verify with Adobe.
I used my Adobe PS license to upgrade to the CS Creative Suite. I used my CS Creative Suite license to upgrade to CS2 Production Studio. When I went to upgrade just my PS to CS3 PS-Extended, I was no longer able to just upgrade that one component, though I had an individual license for PS from the very beginning. I could ONLY upgrade to one of the CS3 suites, and no longer any individual program. This came as a shock to me, but Adobe verified it repeatedly.
Something to check out, and good luck. Also, Adobe might well have changed the suite component policy, from CS3.