  We investigate the pressure and temperature dependence of the lattice
dynamics of the underdoped, stoichiometric, high temperature superconductor
YBa2Cu4O8 by means of Raman spectroscopy and ab initio calculations. This
system undergoes a reversible pressure-induced structural phase transition
around 10 GPa to a collapsed orthorhombic structure, that is well reproduced by
the calculation. The coupling of the B1g-like buckling phonon mode to the
electronic continuum is used to probe superconductivity. In the low pressure
phase, self-energy effects through the superconducting transition renormalize
this phonon, and the amplitude of this renormalization strongly increases with
pressure. Whereas our calculation indicates that this mode's coupling to the
electronic system is only marginally affected by the structural phase
transition, the aforementioned renormalization is completely suppressed in the
high pressure phase, demonstrating that under hydrostatic pressures higher than
10 GPa, superconductivity in YBa2Cu4O8 is greatly weakened or obliterated.
