The only way I can think of to save $$$ in this scenario (Wednesday pick-up/Sunday drop) is to do a one day rental Wed.-Thur. and then do a three day weekend rental Thursday-Sunday. A PITA depending on how far from MKE he's going to be, but w/Wed. pickup Hertz will charge weekday rates for all four days. An example I ran was a midsize for Wed. thru Sun. w/AARP CDP (50075) came out to 35.69+tax/day. You can still do the one day Wed - Thur for 35.69, drop that off and pick up another midsize for Thur-Sun for $21.24/day. The Wednesday pickup is the killer here, you just have to decide if its worth the $45 or so savings to go back to MKE for a dropoff/pickup on Thursday.
Best regards,
JC