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We have investigated the jet-linked \mbox{X-ray} emission from highlyradio-loud quasars (HRLQs; $\log R>2.5$) at high redshift. We studied the X-rayproperties of 15 HRLQs at $z>4$, using new {\it Chandra} observations for sixobjects and archival {\it XMM-Newton} and {\it Swift} observations for theother nine. We focused on testing the apparent enhancement of jet-linked\mbox{X-ray} emission from HRLQs at $z>4$. Utilizing an enlarged (24 objects)optically flux-limited sample with complete X-ray coverage, we confirmed thatHRLQs at $z>4$ have enhanced X-ray emission relative to that of HRLQs at$z\approx$ 1--2 with matched UV/optical and radio luminosity, at a\mbox{4.0--4.6}~$\sigma$ level; the X-ray enhancements are confirmedconsidering both two-point spectral indices and inspection of broad-bandspectral energy distributions. The typical factor of enhancement is revised to$1.9^{+0.5}_{-0.4}$, which is smaller than but consistent with previousresults. A fractional IC/CMB model can still explain our results at highredshift, which puts tighter constraints on the fraction of IC/CMB X-rays atlower redshifts, assuming the physical properties of quasar jets do not have astrong redshift dependence. A dominant IC/CMB model is inconsistent with ourdata.

Investigating the X-ray enhancements of highly radio-loud quasars at z > 4