Why is my 18 mth old son still sleepy?

My son is 18 mths old and sleeps from 8pm till 8:30 am and then takes a nap from 11:30 am till 2:00 pm. By 4pm is is acting all cranky and sleepy. He starts sucking his fingers and grabs his sleep toy, but he won't sleep when I put him in his crib. How can I adjust his schedule so he is content? Thanks for any help!!!!