  GB 1508+5714 is a high-redshift blazar ($z$ = 4.3) and a spectrally soft
$\gamma$-ray source has been detected in its direction. By analyzing 11.4-yr
{\it Fermi}-LAT data, significant long-term variability of the $\gamma$-ray
source is confirmed. More importantly, a $\gamma$-ray emission flare appeared
in an epoch of several tens of days in year 2018, when the flux is about four
times of the value from the global fit. Meanwhile, optical flares displayed in
both $r$- and $i$-bands from the {\it Zwicky} Transient Facility light curves.
Detections of the simultaneous $\gamma$-ray and optical brightening provide a
decisive evidence to pin down the association between the $\gamma$-ray source
and GB 1508+5714, which makes it the {\it first} identified $\gamma$-ray blazar
beyond redshift 4. A broadband spectral energy distribution in the high flux
state is constructed and the origin of the multiwavelength brightening is also
briefly discussed. Upcoming wide-deep-fast optical telescopes together with the
$\gamma$-ray sky surveyors will shed lights on the role that the AGN jets play
in the early cosmic time.
