  4-derivative gravity provides a renormalizable theory of quantum gravity at
the price of introducing a physical ghost, which could admit a sensible
positive-energy quantization. To understand its physics, we compute
ghost-mediated scatterings among matter particles at tree-level, finding a new
power-like infra-red enhancement typical of 4-derivative theories, that we dub
'$ghostrahlung$'. Super-Planckian scatterings get downgraded to Planckian by
radiating hard gravitons and ghosts, which are weakly coupled and carry away
the energy.
