Robert J. La Follette claims that there is a problem with Photoshop 4.0 when you try to save a file as a .gif file using the CompuServe GIF format - using the Save As... option (not the GIF89a Export filter). What may happen is that Photoshop will quit with a Type 3 Error. Robert adds: "I have also noticed that if Photoshop does not suddenly quit, sometimes the file is not even created. A quick scan of a Photoshop newsgroup
confirmed that other users were also having this problem."
However, not everyone has this problem. When I tried it, I could save files with no errors at all.