This issue is sadly very frequent. I had the same problem, someone recommended me to just defend without attacking the production cruiser.
So I started all over, with the farther save I had, and it worked, by the end of the mission I had no Sandskimmer bugged.
But still, I cannot end the mission. No ennemies anywhere (no red circle on sensors, looked everywhere just in case), but the mission is still running and nothing happens. I don’t want to start it all over a third time…
The person who decided to release the game so early despite the probable dev’s protestations should reconsider his decisions.
Here is my save file at the end of the mission 4, without anything remaining, but cannot clear. Hope it will helps debugging : http://pulsahr.free.fr/temp/M4_finished.sbsave
EDIT 2 :
I DID IT !
I don’t know what worked, but I kept a saved when the final scan was at 90%, so I can prepare something before reinforcments arrive. What I usually did :
BEFORE : regroup all troops near the Kapisi, position vehicles so the landscape is a big advantage : narrowed path protected by armored, railgun a little behind, light vehicles far behind ready to make a detour to get the ennemy railguns from behind. Even the production cruiser was wrecked.
AFTER : I regrouped everything, but a bit farther to the “south”, so the ennemy cruiser was able to “enter” the first scan site. Thet did in fact, I lost more ships, but destroyed everything, including the cruiser. A small ennemy regiment started to go west to attack the scan site. I ordered an airstrike on the forst armored vehicle, the (3) turrets destroyed the rest, mission completed.
I don’t know if that’s the strike crafts, the southern position, or the detached ennemy regiment who did the trick, but it worked.