35 ILCS 505/13a.7
Notwithstanding the provisions for credit memoranda, credits or refunds contained in Section 13a.3 of this Act, no credit memorandum, credit or refund shall be allowed or made based upon a return filed more than 4 years after the due date of the return or the date the return is filed, whichever is later. In any case in which there has been an erroneous refund of tax payable under this Section, a notice of tax liability may be issued at any time within 3 years from the making of that refund or within 5 years from the making of that refund if it appears that any part of the refund was induced by fraud or the misrepresentation of material fact. The amount of any proposed assessment set forth by the Department shall be limited to the amount of the erroneous refund.
35 ILCS 505/13a.7