Without having more information, I'd say the short answer is no. Regardless of how the cards are set up, they will exist in an either/or configuration; that is, you can't have the old 4 meg card "pitching in" with the 16 meg card to give you an effective total of 20 megs. At any given time, either the 4 meg Diamond card will be feeding your monitor or the 16 meg Creative Labs card will.
(The only exception here would be if you had two Voodoo 2 cards connected up in an SLI [scan line interleaving] setup, but I'm assuming that neither card here is a Voodoo 2, based on the memory sizes.)
The only other advantage you would THEORETICALLY have is if the 4 meg card is a Voodoo 1 (3dfx) and the Creative card is not. Then the 4 meg 3dfx card would allow you to play certain 3dfx-only games. HOWEVER, the only games that are 3dfx-only are very old and likely not on your hard drive anymore. Also, that 4 meg card will be insufficient to play any newer games anyway because of its limited memory, so it's probably not an issue regardless.
So again, short answer is no. Hope this helps...
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