I don't think that this is really a 4th amendment issue, but what about that "Right of Privacy" that the Left screams about in the abortion context. Supposedly, we all have a "right to privacy" that is one of the penumbras emanating from the Bill of Rights. I think my financial transactions should be private, as should everyone elses.
If we scrapped our current tax code, and had a VAT or a national sales tax, this would no longer be an issue as tax evasion would be nearly impossible, as retailers would collect the Federal tax, just as they do at the state level. Surely this is a better idea than taxing income, wiht all of the attendant snooping involved in having an income tax. But I think the attendant snooping is what our government likes!!