I know from experience that battery acid has a shelf life of one year. Four years? I don't know, I'd replace it if I were you.
How badly you can screw up a piece depends on how you define screw up. I have totally wrecked a real Colt AR15 lower receiver, not by anodizing, but by stripping off the old anodize with lye. The lye enlarged the pin holes to the point the receiver was mechanically useless. Common sense would dictate that you don't attempt anodizing anything valuable until you have the processs down pat by using scrap to practice on.
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