That really is a quick trip, with only four days to see a pretty big state. The Bangor Airport Hotel is a good choice. It's connected to the airport, recently updated, good breakfasts. It's a Four Points Sheraton. It would be worth it to spend a little more to stay right there, especially coming in at midnight. The other motels in the area aren't nearly as nice and involve a drive to get to them.
There's not much to see in Bangor for the day. You should be able to get out of there before noon and make the hour's drive to Bar Harbor. If you get into town before check-in time, stop at the Acadia National Park Visitor's Center to browse the brochures and pick up a map. You might even be able to drive the Loop Road and up to the top of Cadillac Mountain on that first Thursday afternoon.
Look around at www.sleepbarharbor.com to choose a Bed and Breakfast in the area. There are a number of them listed there, with amenitites and links to individual web pages. The Hearthside, Aysgarth Station, and Atlantean are three of the nicer ones. Saltair and Bass Cottage Inn are two of the higher-priced but very nice ones.
On Friday you should be able to rent a bike for the carriage roads, book a kayak tour, have popovers at Jordan Pond House, maybe even take a lighthouse boat tour. Their are easy trails to walk and very difficult trails to hike. Or you can just sit on a rock near Thunder Hole or Sand Beach and watch the waves splash high on the granite shore.
Driving down the coast on Saturday may or might not be a good idea. Just staying on Highway One will give you only coastal glimpses. If you go down the peninsula to Deer Island and Stonington you will see more, but it will take time. You don't mention a place to stay on Saturday night. Camden might be a good overnight stop. But it's only about a two and a half hour drive to Camden, depending on traffic.
On Sunday you will again need to get off of Highway One to see much of the ocean. The towns along One are all on large rivers and don't really have any water view. You might just go on straight to Portland and see the Portland Head Lighthouse and the few others in the area.