The traditional 16 rider 20 heats format might work for the GPs if the order of the heats is rearranged. Number the riders 1-14 based on the previous GP results with the wild cards 15 and 16 and switch what are now heats 1-4 to 16-20 as follows:
Heat 17 - 15,14,16,13
Heat 18 - 10,11,9,12
Heat 19 - 5,7,6,8
Heat 20 - 1,2,3,4
This way you stand a better chance of keeping the meeting alive until the end, whether or not you have semis and a final. These seem pointless in a meeting where everyone rides each other once.