The JCMT Transient Survey: Four Year Summary of Monitoring the
  Submillimeter Variability of Protostars
  We present the four-year survey results of monthly submillimeter monitoring
of eight nearby ($< 500 $pc) star-forming regions by the JCMT Transient Survey.
We apply the Lomb-Scargle Periodogram technique to search for and characterize
variability on 295 submillimeter peaks brighter than 0.14 Jy beam$^{-1}$,
including 22 disk sources (Class II), 83 protostars (Class 0/I), and 190
starless sources. We uncover 18 secular variables, all of them protostars. No
single-epoch burst or drop events and no inherently stochastic sources are
observed. We classify the secular variables by their timescales into three
groups: Periodic, Curved, and Linear. For the Curved and Periodic cases, the
detectable fractional amplitude, with respect to mean peak brightness, is
$\sim4$ % for sources brighter than $\sim$ 0.5 Jy beam$^{-1}$. Limiting our
sample to only these bright sources, the observed variable fraction is 37 % (16
out of 43). Considering source evolution, we find a similar fraction of bright
variables for both Class 0 and Class I. Using an empirically motivated
conversion from submillimeter variability to variation in mass accretion rate,
six sources (7 % of our full sample) are predicted to have years-long accretion
events during which the excess mass accreted reaches more than 40 % above the
total quiescently accreted mass: two previously known eruptive Class I sources,
V1647 Ori and EC 53 (V371 Ser), and four Class 0 sources, HOPS 356, HOPS 373,
HOPS 383, and West 40. Considering the full protostellar ensemble, the
importance of episodic accretion on few years timescale is negligible, only a
few percent of the assembled mass. However, given that this accretion is
dominated by events of order the observing time-window, it remains uncertain as
to whether the importance of episodic events will continue to rise with
decades-long monitoring.
