So happy that you are down to a reasonable class size. No one crazy is good.
Keep those 4 boys moving... seriously, I know you do a good job of this, anyway, but it sounds like no one has given them any boundaries or listens to them, so they just do whatever they feel like doing. I'd tire them out a bit, and at the same time slowly introduce some rules around the blurt outs.
So, while this doesn't come under the area of teaching, I had one of the 3 clients I took with me from my last job have a "tantrum" in my office last week. He was on the verge of destroying stuff, kicking the wall, screaming. He is 10, an adoptive kid, who made some progress with me when I did a trauma based program with him when I went to his house. It's mostly been teaching his parents how to be consistent, respond to this stuff. These people are altruistic, but they never should have adopted a traumatized kid of a different race, who has been in more placements than anyone i have worked with. I have seen this kid destroy, and i mean destroy his classroom, twice (a special ed class). I got him calmed down a bit and when his mom came into the room, he started up again. I know what triggered this, but it was scary, couldn't get him to stop. He is my youngest client; I don't take anyone under 6th grade now, only adolescents and adults. I want to refer him to a trauma center, where they have people who deal more with younger kids, but his mom will be devastated, and so will the kid. But, I feel like i am not helping him at all.
You might not totally stop all of the impulsivity, but I know you can make headway. And, I am betting at least 2/4 have undiagnosed ADHD.