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Adobe has separate lens correction files for 5D and for 1DmkIII, so size does matter.
The files for LR4 are much larger than the files CS4 makes so the LR4 files are probably useless on the CS4.
They have done a good job on the lens correction. It was probably enough more work that the decided to change the policy on supplying plug in files for new cameras.
I have a student copy of CS4. I was originally looking at possibly upgrading because I am looking at upgrading my backup camera from my 20D to either a T3i or T4i and Adobe no longer is supplying the plug in for new cameras for CS4 (what a pain); so if I upgrade I have to buy CS6 from scratch, or buy Lightroom. I am trying Lightroom. I prefer the layout of the Camera Raw with CS4 a lot more because its just cleaner and you can see the picture larger, but I like the new features of Lightroom ( and CS6 I presume) like the lens correction. I may wait until I actually buy a new back-up camera, who knows maybe I'll have enough bucks at the time to buy the CS4 photoshop, or even afford a 7D or 5D.