  Recently for the sine-Gordon equation it has been established that during
collisions of $N$ slow kinks maximal energy density increases as $N^2$. In this
numerical study, the same scaling rule is established for the non-integrable
$\phi^4$ model for $N\le 5$. For odd (even) $N$ the maximal energy density is
in the form of potential (kinetic) energy density. Maximal elastic strain is
also calculated. In addition, the effect of the kink's internal modes on the
maximal energy density is analysed for $N=1, 2, 3$. Our results suggest that in
multi-soliton collisions very high energy density can be achieved in a
controllable manner.
