You can change the lane assignments when the race starts. If you want to force a specific lane entry you have to use one of the two custom rotation types and set it up. For example, "custom round robin sequence" with a value of [1, 2, 3, 0] would not let anybody use lane 4 no matter how many drivers you have. [0, 2, 3, 4] would do the same thing only lane 1 would be ignored. There are other ways to do it but this is one of the easiest, but using the "Modify Heats" option you can always move drivers around on the lanes.