i know that some people are saying that the .psd is a more universal file type and shouldn't have any compatibility problems, but i'm finding that this is not the case for a few files. thanks!
If you have a layered file that contains CS6 features that are not available in CS4, CS4 should just be discarding those features,
If you include a flattened composite ("Maximize File Compatibility"), CS4 should be able to open that.
If you flatten a copy of your file before saving it, CS4 will be able to open that.
I cannot offer any more ideas since you gave us zero details about anything,
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