I'm absolutely clueless when it comes to AR pistol stuff.

The KAK Super Sig SB15 buffer tube says it's made for the old Sig SB15 and looks like it adds a little LOP to the pistol brace setup.

I'm looking at the SBM4 and I'm not sure if diameters and what not are the same (even though it looks like the guys who make the SBM4 now make the SB15).

Any other pistol tube I should be looking at or will the KAK tube work?

It will work but there will be a space of about an inch or so in the end of the brace. The new braces are longer than the originals (I have both)

If you CALL APF Armory they actually make a tube for the new braces that are longer. The one they have listed on their website is also for the SB15, but if you call them they just started making a tube like the super tube for the brace that you want.

http://www.apfarmory.com