Establishing a basis for IRA to Roth IRA conversion?

I took the opportunity to convert about $10k from IRA to Roth IRA because I will be in a lower marginal tax bracket than usual this year. That $10k is now worth about $6k. Is there any way to deduct the $4k in losses from my taxable income resulting from the conversion (I'm thinking something like using 12/31 prices of the mutual funds as the taxable amount)?