Fourth and Goal isn't that hard once you know how it works. When you enter the chalet you trigger two timers. A short timer on the hostages (timed till execution & fail) and a longer one on the nuke (timed till detonation & fail).
1. Neutralise the hostage timer by killing the guys closest to them and controlling that room. Best to breach with two teams (one through the windows, one through the main doors) so you can hit all four walls at once.
The OBJ is only complete when all tangos are dead, but the timer is irrelevant if you've got guns in there. If you leave them alone someone will wander by and cap them so beware!
2. Neautralise the nuke timer by finding it, killing the guys, and picking it up. I have found four rooms so far that he shows up in. Once you know where they are you can use your sensors to make an educated guess and cut through to them pretty quickly.
Its an awesome adrenalin mission! Stealthy and slow for OBJ1 and OBJ2, and super fast for OBJ3 and 4. Anyone else with working strats for it?