I've carried a 4" 66, a bit (i.e., a stainless 19). To me, a 4" sixgun shoots better than it conceals (indeed, it shoots about as well as any handgun there is). With a holster that is well made and fits you well, such a gun can be concealed very tolerably, but it will never be as comfortable as a 2" j-frame (like a 60 or a 640); even the difference between a 2.5 or 3" k-frame and the 4" version of the same gun is significant in my experience.
The 4" k-frame .357 (the model 19/13/66/65) was intended to be an open-carry gun for patrolmen, and was indeed the top of the LE heap for some time. For open carry, it is my absolute first choice for comfort and shootability. Border Patrolman and pistolero Bill Jordan, who had a hand in its design and introduction, would not have considered it very suitable as a primarily concealed weapon. Opinions will vary on this, but in my experience, Mr. Jordan was right.
If you're an experienced revolver shooter (i.e., you can shoot accurately and fairly quickly in double action) looking for something primarily for concealed carry, I'd consider getting a shorter gun. If you're new to revolver shooting, then a 4" k-frame is the perfect place to start and learn--but recognizing that you probably will want to get a smaller carry-gun at some time in the future.