  We argue that type II string theories contain unstable NS4 branes, which
descend from a conjectured unstable M4 brane of M-theory. Assuming that an M2
brane can arise in M5 brane/anti-brane annihilation, the unstable M4 brane, and
also an unstable M3 brane, must exist as sphalerons. We compare the tensions of
the unstable NS4 branes, M4 brane, and related type II unstable D-branes, and
present 11d supergravity solutions for unstable Mp branes for all p. We study
the Z_2 gauge symmetry on the worldvolume of unstable branes, and argue that it
can never be unbroken in the presence of lower brane charge.
