Relationships: I don't know what to do

I'm in a 4 yr relationship with a wonderful, smart,and funny man. He treats me with love and respect and he buys me whatever I want when he can afford it. But lately I just find myself less and less attracted to him. From the first time I met him,I didn't think he was all that attractive but he had such a fun personality. We always had fun together and he always made me laugh. He became my best friend. I don't believe that I will ever meet a man that treat me as good as he does. I don't know what to do because I don't want to brake his heart but at the same time I can't be intimate with him any more. I'm really afraid of being alone and putting myself back out there. And I don't want to lose my best