Sorry, I did not see this one before, I should pay closer attention.

For the "B4" identifier, I went to the USMC Scout Sniper Instructor Course in Quantico VA. We didn't have an ASI for it until fairly recently. It was more challenging than the other schools you mention. it also gives you skills for accomplishing the mission, not just a way to get there. The Army always focused too much on infiltration skills and (in my opinion, until lately) not enough on doing your job once you got there. Sniper, SERE and the CQB schools were probably my favorites, although HALO was a lot of fun also. Most people do not realize how difficult it is to jump from an aircraft at 25,000 + feet AGL in pitch black night, with 200 pounds of equipment strapped to you, stay formed up with your team in free fall and all land within 100 meters of the designated point on the ground. It isn't skydiving.