  Finding reliable indicators of Lyman continuum (LyC) photon leakage from
galaxies is essential in order to infer their escape fraction in the epoch of
reionisation, where direct measurements of LyC flux are impossible. To this
end, here we investigate whether strong C IV $\lambda \lambda 1548,1550$
emission in the rest-frame UV spectra of galaxies traces conditions ripe for
ample production and escape of LyC photons. We compile a sample of 19
star-forming galaxies in the redshift range $z=3.1-4.6$ from the VANDELS survey
that exhibit strong C IV emission, producing a stacked spectrum where all major
rest-UV emission lines are clearly detected. Best-fitting spectral energy
distribution models containing both stellar and nebular emission suggest the
need for low stellar metallicities ($Z=0.1-0.2\,Z_\odot$), young stellar ages
($\log(\rm{age/yr}) = 6.1-6.5$), a high ionisation parameter ($\log U = -2$)
and little to no dust attenuation ($E(B-V)=0.00-0.01$). However, these models
are unable to fully reproduce the observed C IV and He II line strengths. We
find that the Ly$\alpha$ line in the stacked spectrum is strong and peaks close
to the systemic velocity, features that are indicative of significant LyC
photon leakage along the line-of-sight. The covering fractions of
low-ionisation interstellar absorption lines are also low implying LyC escape
fraction in the range $\approx 0.05-0.30$, with signatures of outflowing gas.
Finally, C IV/C III] ratios of >0.75 for a subset of individual galaxies with
reliable detections of both lines are also consistent with physical conditions
that enable significant LyC leakage. Overall, we report that multiple
spectroscopic indicators of LyC leakage are present in the stacked spectrum of
strong C IV emitting galaxies, potentially making C IV an important tracer of
LyC photon escape at $z>6$.
