hmmm....1st of all it wont charge to 100%.i think 97% is the highest reported full charge.
my battery drained only 20% when i watched a film for 1:30hrs... So i hope i will have atleast 50% left after 4hrs.But i havent tested it though.
My brighnness level was 3 with full volume(headphones) Did u watch it with speakers or headphones?