I know I'll get yelled at here, but I'll suggest that the 4M might not be the optimal mouthpiece for lead playing.
A Warburton 4M/7 is my mouthpiece for everything...except lead. It would take too much work for me to get through a gig/rehearsal playing a typical big band lead book.
For lead I really like the Marcinkiewicz Shew mouthpieces. I think the 1.25 or 1.5 are GREAT (the 1 is too shallow for me). (I now use an Austin Custom Brass Lead 1.25 which is similar to the Shew 1.25 but with a softer rim).
A mouthpiece isn't the root cause of your tension issue, but it could be a piece of the puzzle.