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We present a study of C/2019 Y4 (ATLAS) using Sloan $gri$ observations frommid-January to early April 2020. During this timespan, the comet brightenedwith a growth in the effective cross-section of $\left(2.0 \pm 0.1 \right)\times 10^{2}$ m$^{2}$ s$^{-1}$ from the beginning to $\sim$70 d preperihelionin late March 2020, followed by a brightness fade and the comet graduallylosing the central condensation. Meanwhile, the comet became progressivelybluer, and was even bluer than the Sun ($g - r \approx 0.2$) when thebrightness peaked, likely due to activation of subterranean fresh volatilesexposed to sunlight. With the tailward-bias corrected astrometry we found anenormous radial nongravitational parameter, $A_{1} = \left(+2.25 \pm0.13\right) \times 10^{-7}$ au d$^{-2}$ in the heliocentric motion of thecomet. Taking all of these finds into consideration, we conclude that the comethas disintegrated since mid-March 2020. By no means was the split new to thecomet, as we quantified that the comet had undergone another split event aroundlast perihelion $\sim$5 kyr ago, during which its sibling C/1844 Y1 (GreatComet) was produced, with the in-plane component of the separation velocity$\gtrsim$1 m s$^{-1}$. We constrained that the nucleus of C/2019 Y4 beforedisintegration was $\gtrsim$60 m in radius, and has been protractedly ejectingdust grains of $\sim$10-40 $\mu$m (assuming dust bulk density 0.5 g cm$^{-3}$)with ejection speed $\sim$30 m s$^{-1}$ in early March 2020 and increased to$\sim$80 m s$^{-1}$ towards the end of the month for grains of $\sim$10 $\mu$m.

Observations of Disintegrating Long-Period Comet C/2019 Y4 (ATLAS) -- A Sibling of C/1844 Y1 (Great Comet)