VERY tough relationship question. HELP!?

OK...Here's the story...it's long...sorry...I have been dating the same guy for abt 4 yrs now. He's amazing. He's funny, incredibly generous, supportive, devoted, decent in the sack, has the potential to be successful, and we have a lot in common. But...I was never totally attracted to him physically and I feel like part of what I originally loved abt him was his devotion to me. Since then we have been through so much and I feel like I'd be crazy not to want to marry him. But every time I think abt it, I have a ton of doubts. I always end up comparing our relationship to this idea in my head; an idea stemming from a longtime crush that never materialized but I wanted him sooo bad. I have never really felt that "magic" that some people describe. I sometimes think..is that just lust? Is it worth breaking up with a great guy for someone that doesn't exist? Is it normal to feel this lack of chemistry and fantasize abt non-existent "dream" guys that aren't real? Advice...please.