I cannot tell you for certain. The PDF tables I was able to pull up showed me that the serial PC66-222-920 4M had 64MB, but you have PC66-222-910...but still 4M.
So an educated guess would tell me that you have 64MB. But it is not something I am completely certain on.
Notice most of the ones you can buy are 64MB, so I think that it is safe to think it is actually 64MB.
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