Harper, McVey, Hancock and Leadsome were all hard brexiteer candidates, so Boris should pick up many of their 50 votes.
But in the referendum 185 Tories voted to Remain and 139 to Leave, so you would expect the final two to be one Remainer and one Leaver.
Boris is the frontrunner to get the Leave vote, but he has a bad reputation amongst his fellow MPs.