  We report here the discovery of NGC 7793 ULX-4, a new transient ultraluminous
X-ray source (ULX) in NGC 7793, a spiral galaxy already well known for
harbouring several ULXs. This new source underwent an outburst in 2012, when it
was detected by \textit{XMM-Newton} and the \textit{Swift} X-ray telescope. The
outburst reached a peak luminosity of 3.4$\times 10^{39}$ erg\ s$^{-1}$ and
lasted for about 8 months, after which the source went below a luminosity of
$10^{37}$ erg\ s$^{-1}$; previous \textit{Chandra} observations constrain the
low-state luminosity below $\sim$ 2$\times 10^{36}$ erg\ s$^{-1}$, implying a
variability of at least a factor 1000. We propose four possible optical
counterparts, found in archival HST observations of the galaxy. A pulsation in
the \textit{XMM-Newton} signal was found at 2.52 Hz, with a significance of
$\sim3.4\,\sigma$, and an associated spin-up of $\dot{f} = 3.5\times10^{-8}$
Hz.s$^{-1}$. NGC 7793 is therefore the first galaxy to host more than one
pulsating ULX.
