First of all, thank you for an amazing campaign.
I'm playing this campaign on DCS World 2.5 and cannot tell wether the problem persists in previous versions, or if it has to do with the upgrade to 2.5.
My two first attempts on mission 4 ended with the Chinook crashing in the large warehouse/building in the AO when landing. On the third try I demolished this building with two jdam's before the chopper was called in. This time it appeared that the crash was delayed enough to trigger the mission accomplished/RTB and I was able to finish the mission.