  We report results of a spectropolarimetric and photometric monitoring of the
weak-line T Tauri star LkCa4 within the MaTYSSE programme, involving ESPaDOnS
at the Canada-France-Hawaii Telescope. Despite an age of only 2Myr and a
similarity with prototypical classical T Tauri stars, LkCa4 shows no evidence
for accretion and probes an interesting transition stage for star and planet
formation. Large profile distortions and Zeeman signatures are detected in the
unpolarized and circularly-polarized lines of LkCa4 using Least-Squares
Deconvolution (LSD), indicating the presence of brightness inhomogeneities and
magnetic fields at the surface of LkCa4.
  Using tomographic imaging, we reconstruct brightness and magnetic maps of
LkCa4 from sets of unpolarized and circularly-polarized LSD profiles. The
large-scale field is strong and mainly axisymmetric, featuring a ~2kG poloidal
component and a ~1kG toroidal component encircling the star at equatorial
latitudes - the latter making LkCa4 markedly different from classical TTauri
stars of similar mass and age. The brightness map includes a dark spot
overlapping the magnetic pole and a bright region at mid latitudes - providing
a good match to the contemporaneous photometry. We also find that differential
rotation at the surface of LkCa4 is small, typically ~5.5x weaker than that of
the Sun, and compatible with solid-body rotation.
  Using our tomographic modelling, we are able to filter out the activity
jitter in the RV curve of LkCa4 (of full amplitude 4.3km/s) down to a rms
precision of 0.055km/s. Looking for hot Jupiters around young Sun-like stars
thus appears feasible, even though we find no evidence for such planets around
LkCa4.
