The "official" cache settings have always been 128k per megabyte of RAM. Commonly used items of the OS are stored there rather than on the hard disk, theoretically making the operations faster to execute. You must have about 128mb of ram to get a 4mb cache file, yes? I have always set my Mac that way and find it runs faster this way.
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Macs Rule..if they don't then the Blues do...: )