Greetings,

I am a Canadian and have been working in the US on a G4 visa since 1999. I recently discovered that this makes me not just a 'resident alien,' but a 'non-resident alien' (a concept that still baffles me). While certain people with G4's do not have to pay income tax at all, I do, given my place of employment. But it looks like I have been submitting the wrong tax form (1040EZ). I plan to contact the IRS to deal with the problem, but would like to know what my legal/financial situation is, and what I should do before contacting the IRS, if anything (tax lawyer, perhaps?). Also, is the shift to 1040NREZ going to alter the amount owed? And what about state taxes (New York)? Does that form change as well?

Many thanks

The G-4 visa is nonimmigrant visa for foreign officers or employees of international organizations. You don't have to pay income tax while on G status as you work for international organizations such as the World Bank.
Why do you think that your income is subject to Income tax?
Also on G4, you are exempt from residency. It also means that the income is not subject to FICA taxes.

If you filed resident tax return and want to file non-resident tax return, then you must complete Form 1040X and attach properly done non-resident tax return. Read: Your U.S. Tax Return: U.S. Tax Filing Requirements for Non-Residents

You DO pay income taxes, but to CANADA, not the U.S.

In order to get the taxes back, you will need to file a Form 1040X with a properly-prepared Form 1040NR attached and citing the appropriate article of the U.S.-Canada tax treaty to get the paid taxes back from the IRS.

To get the FICA taxes back, you will need to file Form 843.

I assume NY state recognizes the provisions of the tax treaty. If so, you will need to file Form IT-203X.

Suggest you get professional help doing all this.