Hi, m295763 -- it's great that you're thinking about these issues! You can take a deep breath, though -- the situation is not as dire as it seems.
First, we've found over the years that the IRS is not looking to "go after" small parent groups who weren't following the letter of the law. Instead, they want to see that you're turning your attention to doing things right from this point forward.
The most straightforward course of action is to incorporate in your state as a nonprofit, then file with the IRS for 501(c)(3) tax-exempt status. The very act of incorporating creates a brand-new organization in the eyes of the IRS -- one with no financial history at all. In this way you'll be starting with a clean slate, and the past four years will not be an issue. The fact that you have a bank account and bylaws already puts you ahead of the game from an organizational standpoint.
You *do not* need to hire an attorney or a CPA -- many, many parent groups have successfully filed for tax-exempt status without doing so. You can choose to consult with one, of course! But it's not a must-do.
Here's an article that lays out the process step by step -- it also has some links at the end to additional resources, including our Startup Toolkit which has line-by-line instructions for completing the IRS tax-exemption application (Form 1023).
Tax-Exemption and Nonprofit Status: What PTOs Should Know
Good luck, and let us know how it goes!
Lani @ PTO Today