What is the value a 2004 US silver quarter?

All circulating quarters dated 1965 and later are made of copper-nickel. They don't contain any silver, so they're only worth 25 cents if you found it in change. All of these coins have a P or D mint mark next to the tail of Washington's wig

If your quarter has an S mint mark instead of a P or D and it doesn't have the normal copper core (check its edge) you may have a specially-made "Prestige Proof" coin. Prestige Proof dimes, quarters, and half dollars are made from the same 90% silver alloy used in those denominations before 1965.

To have its full value, a Prestige Proof coin should be in its original or other protective packaging. As of 09/2011 prices run from around $8 up to $15 depending on quality. If the coin was removed from its holder and was spent, its value as a collectible has been mostly destroyed but it's still worth about $8 for its silver content.