I was waiting since 1965 for the promised double action .45 that Smith & Wesson promised that they were working on. Finally in about 1985 it finally came out on the market. I was thrilled and bought the first one I could find for sale. I found that the 645 was the most reliable and accurate .45 that I had ever shot. It would digest the wide CCI hollow points that I have never seen a colt 1911 reliably fire. I think that the 645 is a winner. The 4506 is suspossed to have a better trigger but I like the solid feel of the stainless steel front and backstrap of the 645 in my hand.