Mike is right, but to add some clarity, what seems to have you confused is how the 26 inches versus 16 inches operates. One is minimum overall length and one is minimum barrel length. If you're under EITHER one, it's an SBR.
So:
Rifle 1:
17 inch barrel
27 inch overall
It's a regular rifle.
Rifle 2:
15 inch barrel
27 inch overall
It's an SBR.
Rifle 3:
15 inch barrel
24 inch overall
It's an SBR.
Rifle 4:
17 inch barrel
24 inch overall
It's an SBR.
If both measurements aren't over the minimum, it's a short-barreled rifle, even if the barrel isn't all that short.
Aaron