  Studies of the most distant AGNs allow us to test our current understanding
of the physics present in radio-jetted AGNs across a range of environments. The
decrease in apparent luminosity with distance is the primary difficulty to
overcome in the study of these distant AGNs, which requires highly sensitive
instruments. Our goal is to employ new long wavelength radio data to better
parametrise the broad-band SED of GB 1508+5714, a high-redshift (z=4.30) AGN.
Its high redshift, high intrinsic luminosity, and classification as a blazar
allow us to test emission models that consider the efficient cooling of jet
electrons via inverse Compton losses in interactions with the dense CMB photon
field at high redshifts. A significant detection of this effect in GB 1508+5714
may partly explain the apparent sparsity of high-redshift radio galaxies in
wide-field surveys; detections of this kind are only becoming possible with the
current generation of SKA precursors. We used international LOFAR telescope to
image the long wavelength radio emission around the high-redshift blazar GB
1508+5714 on arcsecond scales at frequencies between 128 MHz and 160 MHz. This
allowed us to compare the spatially resolved structure with higher frequency
observations, and to construct spectral index maps. The LOFAR image shows a
compact unresolved core and two resolved emission regions around 2 arcsec to
the east and to the west of the radio core. We find structure consistent with
previous VLA observations, as well as a previously unreported emission region
to the east. We interpret the arcsecond-scale radio structure of GB 1508+5714
as a FR II-like radio galaxy at a small viewing angle. Our SED modelling shows
that a scenario featuring significant quenching effects caused by interaction
with the CMB provides a good description of the data, and notably explains the
suppressed radio emission.
