  Exoplanets orbiting pre-main sequence stars are laboratories for studying
planet evolution processes, including atmospheric loss, orbital migration, and
radiative cooling. V1298 Tau, a young solar analog with an age of 23 $\pm$ 4
Myr, is one such laboratory. The star is already known to host a Jupiter-sized
planet on a 24 day orbit. Here, we report the discovery of three additional
planets --- all between the size of Neptune and Saturn --- based on our
analysis of K2 Campaign 4 photometry. Planets c and d have sizes of 5.6 and 6.4
$R_\oplus$, respectively and with orbital periods of 8.25 and 12.40 days reside
0.25% outside of the nominal 3:2 mean-motion resonance. Planet e is 8.7
$R_\oplus$ in size but only transited once in the K2 time series and thus has a
period longer than 36 days, but likely shorter than 223 days. The V1298 Tau
system may be a precursor to the compact multiplanet systems found to be common
by the Kepler mission. However, the large planet sizes stand in sharp contrast
to the vast majority of Kepler multis which have planets smaller than 3
$R_\oplus$. Simple dynamical arguments suggest total masses of $<$28 $M_\oplus$
and $<$120 $M_\oplus$ for the c-d and d-b planet pairs, respectively. The
implied low masses suggest that the planets may still be radiatively cooling
and contracting, and perhaps losing atmosphere. The V1298 Tau system offers
rich prospects for further follow-up including atmospheric characterization by
transmission or eclipse spectroscopy, dynamical characterization through
transit-timing variations, and measurements of planet masses and obliquities by
radial velocities.
