My two cents.
Do K4. Get heavy time and the experience. You'll probably get the 767 with your background, but maybe the 747. I think you will enjoy it and it's a great experience that you can't duplicate with the regionals, but if you don't like it you can always do the move back to direct hire captain. And be a better pilot in the process. Too often I see pilots "pigeonholing" themselves, not branching out and getting the breadth of aviation experiences that might save your bacon some day.
Good luck with your interview and hopefully a decision. This is a good problem to have.