  We use Keck/NIRSPEC to survey a sample of of young ($<$1 Gyr), short period
mini Neptunes orbiting nearby K dwargs to measure their mass loss via the
metastable helium line. We detect helium absorption from all four of the
targets in our initial sample. The first detection, around TOI 560b, was
announced in a previous paper. We now announce three additional detections
around TOI 1430.01, 2076b, and TOI 1683.01. All four planets show an average
in-transit excess absorption of 0.7--1.0%. However, the outflows differ in
their kinematic properties. TOI 1430b exhibits pre-ingress absorption, while
TOI 2076b's outflow is exceptionally optically thick and shows significant
post-egress absorption. For all four planets, the width of the measured helium
absorption signal is consistent with expectations for a photoevaporative
outflow (10--30 km/s, 5000--10,000 K). Unless broadening mechanisms other than
thermal velocity and the bulk outflow velocity are significant, our
observations disfavor core-powered mass loss models, which predict much slower
(1--3 km/s) outflows. We utilize both an isothermal Parker wind model and an
order-of-magnitude method to estimate the mass loss timescale, and obtain
$\sim$ a few hundred Myr for each planet. We conclude that many, if not all, of
these planets will lose their hydrogen-rich envelopes and become super Earths.
Our results demonstrate that most mini Neptunes orbiting sun-like stars have
primordial atmospheres, and that photoevaporation is an efficient mechanism for
stripping these atmospheres and transforming these planets into super Earths.
