Assuming you want to convert CMYK to CMYK, you could make sure the source profile is assigned, change your default CMYK to the destination profile, then cycle the document color mode (File > Document Color Mode) to RGB and back to CMYK. This would be slightly less accurate and probably introduce a small amount of extra quantization than a "direct" conversion, as it will use your default RGB profile as an intermediary. The round-trip performance and gamut of your default RGB profile will affect the accuracy. You would likely get the best result using ProPhoto RGB.
If you want to convert CMYK to RGB or RGB to CMYK, then just simply change the document color mode. If you want RGB to RGB, then you'll probably suffer some noticeable gamut reduction if you use any standard CMYK profile as your default.
I've barely looked at CS4 - there may be an alternative way.
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