A hyper luminous starburst at z=4.72 magnified by a lensing galaxy pair
  at z=1.48
  [Abridged] We discovered in the Herschel Reference Survey an extremely bright
IR source with $S_{500}$~120mJy (Red Virgo 4 - RV4). Based on IRAM/EMIR and
IRAM/NOEMA detections of the CO(5-4), CO(4-3), and [CI] lines, RV4 is located
at z=4.724, yielding a total observed L$_{IR}$ of
1.1+/-0.6x0$^{14}$L$_{\odot}$. At the position of the Herschel emission, three
blobs are detected with the VLA at 10cm. The CO(5-4) line detection of each
blob confirms that they are at the same redshift with the same line width,
indicating that they are multiple images of the same source. In Spitzer and
deep optical observations, two sources, High-z Lens 1 (HL1) West and HL1 East,
are detected at the center of the three VLA/NOEMA blobs. These two sources are
placed at z=1.48 with XSHOOTER spectra, suggesting that they could be merging
and gravitationally lensing the emission of RV4. HL1 is the second most distant
lens known to date in strong lensing systems. The Einstein radius of the
lensing system is 2.2"+/-0.2 (20kpc). The high redshift of HL1 and the large
Einstein radius are highly unusual for a strong lensing system. We present the
ISM properties of the background source RV4. Different estimates of the gas
depletion time yield low values suggesting that RV4 is a SB galaxy. Among all
high-z SMGs, this source exhibits one of the lowest L$_{[CI]}$ to L$_{IR}$
ratios, 3.2+/-0.9x10$^{-6}$, suggesting an extremely short gas tdepl of only
14+/-5Myr. It also shows a relatively high L$_{[CI]}$ to L$_{CO(4-3)}$ ratio
(0.7+/-0.2) and low L$_{CO(5-4)}$ to L$_{IR}$ ratio (only ~50% of the value
expected for normal galaxies) hinting a low density of gas. Finally, we discuss
that the short tdepl of RV4 can be explained by either a very high SFE, which
is difficult to reconcile with major mergers simulations of high-z galaxies, or
a rapid decrease of SF, which would bias the estimate of tdepl toward low
value.
