Answer
$ \sqrt{15}\;mc$
Work Step by Step
We know that the total energy of a particle is given by
$$E^2=(mc^2)^2+(pc)^2$$
and we are told that $E=4 mc^2$, so
$$(4 mc^2)^2=(mc^2)^2+(pc)^2$$
Hence,
$$p^2c^2=16( mc^2)^2-(mc^2)^2=15m^2c^4$$
$$p^2 = 15m^2c^2$$
$$\boxed{p = \sqrt{15}\;mc}$$
