Although Kara Thrace, Starbuck, has seemingly returned from the dead with the news she's been to Earth and back, everyone believes she's really a Cylon, even Commander Adama. And the four crew members who have just discovered they are the hidden Cylons have to come to grips with that. For the majority this doesn't change who they are. Lee Adama, Apollo, had quit being part of the military and is now part of the ruling body, the Quorum of Twelve. He has lots of dealings with the corrupt Tom Zarek, who has his eye on the Presidency. Meanwhile, the reinstated President Laura Roslin's cancer has returned.
There's a revolution among the Cylons and half of them side with the humans. This was all sparked when Cavil began to lobotomize the Cylon Raiders so they couldn't think for themselves. The revolution is led by Caprica Six, the same one who was originally with Baltar in the very first Galactica mini-series. In The Hub, these Cylons and the Galactica work together to destroy the last resurrection ship. Once this is done when a Cylon dies they'll be dead forever. This ups the stakes. A lot of humans don't like this treaty with the Cylons, particularly Zarek and Gaeta, who instigate a mutiny aboard the Galactica. Tigh and Adama are almost killed but they manage to regain the ship with Starbuck's help. And the two traitors are executed.
Midway through the season they discover Earth, but it's been destroyed a few thousand years previously. There are plants but the radioactivity is high and no one could survive the radiation. The weirdest thing of all is that Starbuck finds her own corpse, crashed in a Viper. Did she really die at the end of Season Three and was resurrected? Is she human or a Cylon? Being on this planet triggers flashbacks for the Final Five, who remember living on this very world. Adama makes the decision that they have to move on and find a habitable planet. But Galactica is falling apart from age and the stress the ship has been through and Chief Tyrol convinces Adama that the only way to make her last longer is to repair it with Cylon technology, basically making the ship a hybrid. They only have a few jumps left. That is if Cavil and his Cylons don't kill them all first. So a huge sacrifice is made to destroy the majority of the Cylons. It's epic.
The show does have a great ending, though it's sad for many of the pivotal characters. And yes, the majority of the questions are answered. The only series I've watched that has had a better ending was SIX FEET UNDER.