One More Four

Regarding Dave Mustaine thinks there ought to be another Big Four show prior to Slayer's retirement, in my opinion, if there is going to be another Big Four show, it should be held in Chicago, at Chicago Open Air 2020. That would be perfect (unless it is already being planned for Chicago Open Air 2019 in May 2019 - the lineup is being announced tomorrow.)

Editor's Note: Nope, there won't be a Big Four show at Chicago Open Air 2019, per these tour dates:
Source: Slayer Announce More North American Farewell Tour Dates; We’re Giving Away Tickets to Every Stop! December 10th, 2018 at 10:00am:
5/2/19 Phoenix, AZ Ak-Chin Pavilion
5/3/19 Albuqueque, NM Isleta Amphitheater
5/5/19 El Paso, TX UTEP/Don Haskins Center
5/7/19 McAllen, TX Bert Ogden Arena
5/8/19 Irving, TX The Pavilion at Toyota Music Factory
5/10/19 Tampa, FL MidFlorida Credit Union Amphitheatre
5/11/19 West Palm Beach, FL Coral Sky in West Palm Beach
5/13/19 Huntington, WV Big Sandy Superstore Arena
5/14/19 Columbia, MD Merriweather Post Pavilion
5/16/19 Indianapolis, IN Ruoff Home Mortgage Music Center
5/17/19 Bonner Springs, KS Providence Medical Center Amphitheater

Chicago Open Air 2019 is 5/18 & 5/19 so it would have to happen on Saturday, May 18

5/19/19 Clarkston, MI DTE Energy Music Theater
5/20/19 Youngstown, OH Coveli Centre
5/22/19 Ottawa, ON Canadian Tire Centre
5/24/19 Camden, NJ BB&T Pavilion
5/25/19 Mansfield, MA The Xfinity Center