[CTcard]

... since he's had the most successful first 4 years of any NFL head coach,

I don't think that is quite true, depending upon your definition.

Harbaugh 44-19-1; 69%
George Allen 40-13-3; 74%
George Seifert  52-12; 81%
Don Shula  39-16-1; 71%
Paul Brown 40-8;  83% 
There may be a few others.

On the other hand, he may have walked into the toughest situation of the lot, and by that definition be the most successful.

[CTcard]

... since he's had the most successful first 4 years of any NFL head coach,

I don't think that is quite true, depending upon your definition.

Harbaugh 44-19-1; 69%
George Allen 40-13-3; 74%
George Seifert  52-12; 81%
Don Shula  39-16-1; 71%
Paul Brown 40-8;  83% 
There may be a few others.

On the other hand, he may have walked into the toughest situation of the lot, and by that definition be the most successful.