A lot of action here! 

Won't cost you much and may scare them into action.

I have spent $50,0000 upgrading the home to current, high level finishes, addressed any noted safety concerns by my licensed electrician,  and they signed for home accepting as is.The email was aggressive in nature with no real actionable items or specific concerns other than "GRAVE DANGER". 

Check out the ultimate Beginners Guide by Brandon Turner, be active on the forums, listen to the podcasts, read the books and take action!

Mistakes happen get it figured out before they file a eviction action for non-payment.

***You can read and listen to all of the literature you want, but unless you take action, you will not know the real mechanics of what is between phase 1 of a project to phase 2 of a project and all the way to completion.

I did the my homework on the property including running a prelim title search so a Quite Title action would have been obtainable.

As a 4 plex gives you that extra unit which can boost your cashflow a bit, getting started and taking action is much more important I feel.

The same action applies when looking at the taxes, insurance, PM cost, Vacancy rate, Cap ex, Maintenance, lawn care, routine maintenance etc.... expenses.  

An Illinois landlord would be under zero obligation to re-negotiate anything, and could treat any failure to comply with the terms as a breach, subject to relief from a forcible entry and detainer action (Eviction).