I'm not sure if CS4 supports the 7D Raw files or not, but as was said, it's easy to check for an update -- start Bridge and go to Help/Updates and the utility will check for you.
A couple of brief points:
First, you are mistaken about Digital Photo Professional (DPP) -- it has two ways of producing a "Photoshop-friendly" image. After you are done with whatever processing you want to use DPP for, you can use the Tools/Transfer to Photoshop function and it will open the image in Photoshop as a "tiff" although a file won't be saved unless and until you Save it in Photoshop.
Or, you can use the DPP File/Convert and Save function and DPP will give you a few choices for jpeg and or tiff file type.
What you need to understand about DPP is that it was designed to process/develop Raw files, and as such does not "write over" your Raw images, so that doing a "Save" does not work the way it does with normal image editors. The way of getting images converted into usable image files is either through Convert and Save, Batch processing, or Transfer to Photoshop.
One other method of bringing a Raw image into CS4 is to use the Adobe DNG converter and convert your Raw CR2 files to DNG (digital negative) files. The DNG format was designed by Adobe to be a "universal" Raw file format. There are only a few Raw apps out there that support the DNG format (DPP does not, for example) but the Adobe Camera Raw plugins as well as Lightroom will support it.
So, if the CR4 ACR update doesn't work with your 7D files (and it certainly won't work with the newest cameras) what you will want to do is find the Adobe DNG converter for your CS4 version of Camera Raw -- I believe it's 5.x. So you will convert to a file that is CS4/ACR5.x-compatible.
Know, though, that a 5.x-compatible will not work with your CS2 ACR...