I believe that missions 4, 5, and 6 can be done in any order and that I have done 4A and 6A.
In mission 6A, you attack the Gleipnir, the opposing enemy army's flying fortress superweapon, which has energy weapons that can disintegrate entire fleets of ships, and it has stealth camouflage. After it nukes some of your ships, you track it down and destroy its ability to cloak. Then you must destroy its defenses, guns on the wings, and the SWBMs. These SWBMs are some sort of ballistic missile that destroys all enemies above the Gleipnir. It fires them periodically after you destroy its camouflage ability, and if you are above the Gleipnir when they detonate, you are instantly destroyed.
The key to this mission is maintaining proper altitude. You cannot attack the Gleipnir consistently from below when it can cloak and is using its energy weapon, since anything below it is disintegrated. You must hit-and-run from above when you attack the SWBMs, since you have to be below it when those missions detonate.
After several milk runs (as the commander called them in Steel Talons on the Lynx), this was a really challenging mission. What distresses me right now is that I can't tell, as with the first mission, whether I could have reduced the casualties on my side or not. The first Gleipnir attack on friendly ships during a mission could potentially be prevented if you could get into position quickly enough to destroy the camouflage. I don't think I can do that with my current aircraft, but perhaps I'm just not good enough either.