A noninvasive, compact laserwire system has been developed to measure the transverse emittance of an H− beam and has been demonstrated at the new LINAC4 injector for the LHC at CERN. Light from a low power, pulsed laser source is conveyed via fiber to collide with the H− beam, a fraction of which is neutralized and then intercepted by a downstream diamond detector. Scanning the focused laser across the H− beam and measuring the distribution of the photo-neutralized particles enables the transverse emittance to be reconstructed. The vertical phase-space distribution of a 3 MeV beam during LINAC4 commissioning has been measured by the laserwire and verified with a conventional slit and grid method.