I think Lance Turbo gave you a good explanation, but I’m going to disagree a little bit on who “owns” the rights to tax returns and why it is different for Trump:
One of the most common mistakes I see a lot of people make when looking at these many investigations into Trump and his alleged misdeeds is that he enjoys the same rights and protections as a private citizen. He does not. He is a public servant. As such, he forfeits many rights he may enjoy solely as a private citizen.
As a private citizen, you have a right to expect your tax returns will be kept private except in very limited, specific circumstances. For example, courts at the state level frequently order tax records disclosed in domestic cases so parties can examine income for purposes of determining spousal and/or child support.
But if you think about it, you will likely identify many situations where public servants must forfeit their individual privacy rights. Every time a public servant undergoes a background check, a lie detector test, a check of their credit – and yes, a check of their tax records, they have agreed to undergo such examination in exchange for the privilege of serving the public good. They have also agreed to carry out the laws of the land – irrespective of whether they privately agree with those laws.
When your oath requires you to defend and protect against all enemies, foreign and domestic, and to faithfully uphold the laws of the land as they are written, you’re agreeing that The People have the right to check up on you to ensure you’re carrying out your oath if they believe they have cause.
I will add that the Fourth Amendment is generally construed to mean a protection against an illegal search or seizure. There’s nothing illegal about this search. It’s enshrined in law as an exception for the purposes of oversight.
If you think that the Founders intended for a public servant holding the most powerful office in the land is automatically exempt from oversight because he is claiming rights to which he is forfeit as a private citizen, I’d gently remind you that the Constitution as a whole was written to explicitly prevent this sort of authoritarianism.
Trump is not a private citizen. It’s a mistake to view his position otherwise.