04 would need to go below 06. 06 glaze matures at a lower temp than 04. I'm not sure how well it will turn out. I would test it before putting it on anything important.
You can, however the additional firing might change the color of the first glaze.
You can do all of the above, but I would do sample firings on a small tile or something similar to get a better idea of what the actual outcome will be. Just remember that 04 is a higher heat than 06. If you are using Duncan Metallic glazes, read the labels as the firing instructions are very specific!
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