Mike,
I am going on the expectation that your Association is incorporated as a non-profit (most are) within the State of IL. Expecting this to be the case,
your Association will be complying with the
IL General Not For Profit Corporation Act .
Per
805 ILCS 105/108.15 your board may still conduct business providing a quorum is reached at each board meeting. A Quorum is to be a majority of the directors then in office. Therefore, if you only have 4 directors, 3 would constitute a quorum.
Per
805 ILCS 105/108.30 defers to the Association governing documents about the filling of vacancies. If your documents are silent, IL law uses the word "may" be filled by the Board.
I suspect that your Association also uses the word "may". Therefore, if no-one wants to volunteer from the membership, the Board may leave the seat vacant or may fill the seat with someone from outside the membership (unless your documents prohibit such appointments by having qualification requirements for directors).
Hope this helps,
Tim