We report a new measurement of the diffuse flux of high energy extraterrestrial neutrinos from the entire sky with energies of ${\rm O}(1\,{\rm{TeV}})$ and above. We have analyzed four years of IceCube data recorded from 2012-2015 focusing on neutrino-induced particle showers. These cascades stem predominantly from electron and tau neutrino interactions, provide good deposited energy resolution and have a lower rate of atmospheric production than muon neutrinos. A new event selection has been developed combining traditional straight cuts with gradient boosted multi-class decision trees to isolate cascades more efficiently, resulting in the largest cascade sample obtained by IceCube to date. The observed astrophysical component dominates at energies above $20\,\rm{TeV}$ and is well described by a single, unbroken power-law. The preliminary fit result is a spectral index of $\gamma=2.48\pm0.08$ with a per-flavor normalization at $100\,TeV$ of $\phi=\left(1.57^{+0.23}_{-0.22}\right)\cdot10^{-18}\,GeV^{-1}s^{-1}sr^{-1}cm^{-2}$ in agreement with previous IceCube measurements. We investigated the possibility of a spectral hardening at the upper end of the spectrum by allowing a second power-law component to enter our flux model. No evidence for such hardening has been found. In the near future we expect improved results by adding IceCube's existing cascades from the preceding two years (2010-2011) into this analysis thereby enlarging the total live time to six years.