  Multifrequency Very Long Baseline Array (VLBA) observations were performed to
study the radio morphology and the synchrotron spectra of four high frequency
peaking radio sources. They are resolved in several compact components and the
radio emission is dominated by the hotspots/lobes. The core region is
unambiguously detected in J1335+5844 and J1735+5049. The spectra of the main
source components peak above 3 GHz. Assuming that the spectral peak is produced
by synchrotron self-absorption, we estimate the magnetic field directly from
observable quantities and in half of the components it agrees with the
equipartition field, while in the others the difference exceeds an order of
magnitude. By comparing the physical properties of the targets with those of
larger objects we found that the luminosity increases with the linear size for
sources smaller than a few kpc, while it decreases for larger objects. The
asymmetric sources J1335+5844 and J1735+5049 suggest that the ambient medium is
inhomogeneous and is able to influence the evolution of the radio emission even
during its first stages. The core luminosity increases with the linear size for
sources up to a few kpc, while it seems constant for larger sources suggesting
an evolution independent from the source total luminosity.
