  It has long been known that luminous, ultra-steep spectrum radio sources are
preferentially associated with massive galaxies at high redshifts. Here we
describe a pilot project directed at such objects, to demonstrate the
feasibility and importance of using LOFAR to study the most distant forming
massive galaxies and protoclusters. We have successfully imaged four
high-redshift ($z>2$) high-luminosity radio galaxies with sub-arcsecond
resolution, at 144 MHz, using the International LOFAR Telescope (ILT). Our
targets were 4C 41.17 ($z=3.8$), the "Anthill", B2 0902+34 ($z=3.4$), 4C 34.34
($z=2.4$) and 4C 43.15 ($z=2.5$). Their low-frequency morphologies and the
spatial distributions of their low-frequency spectral indices have been mapped,
and compared with available optical, infrared, and X-ray images. Both for the
Anthill at $z = 3.8$ and B2 0902+34 at $z=3.4$, the location of the steepest
radio emission coincides with the Ly$\alpha$ emitting ionized gas halo. Our
pilot project demonstrates that, because of its outstanding sensitivity and
high angular resolution at low frequencies, the ILT is a unique facility for
studying the co-evolution and interaction of massive galaxies, galaxy clusters,
and supermassive black holes in the early Universe.
