I'd agree with getting the monorail station by 3:45. Grand Floridian would be the last stop around from the Magic Kingdom on the resort monorail. You'll stop first at the Contemporary, then TTC, then Poly, then Grand Floridian.
Unless you really want to take the monorail, you could instead take the launch (boat) from the Magic Kingdom directly to the Grand Floridian. Its a nice ride too.
Either are great options, heck, you're at DisneyWorld!