Can't say why only those four. Two possible reasons is that:
They died when the Free Folk attacked Castle Black at the end of s06e02 "Home".
No one could identify who they were.
Obviously, if they were around to be found, they would be hanging as well.
As for the hanging vs beheading, hanging is the de facto method of execution for traitors, which those four were. They broke their oath (Hence the title "Oathbreaker"). Ser Alliser himself explains in an interview:
Vulture.com: And a brutal one. You might have expected Jon Snow to behead them, as he did Janos Slynt, but to hang them? Compare that to Ned Stark's execution — that quick, clean death now seems to be the mercy Joffrey said it was.
Owen Teale, Ser Alliser's Actor: I asked them, "Why isn't there a bigger drop, when you hang them?" And they said, "Because if it's a bigger drop, you would die instantly. The whip action would snap your neck, and that's it. You would lose consciousness immediately." But this was actually more cruel, more of a punishment, because your feet almost touch the ground, you haven't fallen that far, so there's still some life in you and your body can't help but fight. I said, "That's really gruesome."
As I remember, the speech that I give to Jon Snow, explaining the way I've lived my life — "I fought, I lost, and now I will rest" — he seems at peace about that. But when the moment comes, and you're dangling, you can't help but fight to live. It's important to see them suffer before they actually die, because as Jon Snow sees it it's insurrection. It's treason, what they've committed. It's important to see them suffer, as a message to anybody else who might be thinking of doing the same.
So basically, beheading is the "nice" execution method in Game of Thrones, while hanging is the "not so nice" method. And Traitors/Treason get the "not so nice" method. While you can argue that beheading is enough to make people think twice, something as cruel as a slow painful death of asphyxiation via hanging (instead of neck snapping) may be a better deterrent.
Examples of beheading being Ned Stark. In both manners. Ned is not the type of man that will needlessly make a man suffer. He uses a quick beheading because he has to do it, but he doesn't need to be malicious. And Ned's own death, via beheading, was a "gift" of "mercy" by king bastard Joffery to his intended, Sansa. A quick painless death.