How do I open a 2004 .tax file?

No, the only way you can open a 2004 .tax file would be if you have the 2004 TurboTax software. TurboTax for a different year won't do it. 

Unfortunately, you can't get TurnoTax software from 20 years ago any more. 

If this is a general question about the return that you filed, you may be able to get a copy of your 2004 tax information from the IRS by filing Form 4506-T. Follow this link for more about that. 

There is a service that is NOT affiliated with TurboTax that might be able to help you and print the return for you.  We do not know what it costs.   Try taxprinter.com