This paper describes work in progress on Rocky IV, a prototype microrover designed to demonstrate proof-of-concept for a low-cost scientific mission to Mars. Rocky IV is currently the baseline design for the MESUR mission scheduled for launch in 1996. Rocky IV uses a behavior-based control architecture which implements a large variety of functions displaying various degrees of autonomy, from completely autonomous navigation to very precisely described actions resembling classical AI operators. The control system integrates information from infrared proximity sensors, proprioceptive encoders which report on the state of the articulation of the rover's suspension system and other mechanics, a homing beacon, a magnetic compass, and contact sensors. The robot is able to perform a variety of useful tasks, including soil sample collection, removal of surface weathering layers from rocks, spectral imaging, instrument deployment and sample return, under realistic mission-like conditions in Mars-like terrain.