This paper presents a progress towards the development of the LiquidPiston 'X4' 30kW heavy fueled rotary Compression Ignition engine prototype. The X4 Engine is the newest version of the LiquidPiston unique rotary 'X' engine architecture, which can be likened to a Wankel type rotary engine that has been inverted. The engine operates on an optimized thermodynamic cycle, called the High Efficiency Hybrid Cycle (HEHC), which combines: 1) a high compression ratio; 2) constant volume combustion, effected by a dwell in combustion volume for a significant period of time; and 3) over-expansion, e.g. a greater expansion ratio compared to compression ratio. The engine development is funded by DARPA, and the long term objectives of the program target: 45% brake thermal efficiency; > 1 hp / lb operating in CI of heavy fuel; and the 30kW (40hp) engine will fit in a 10" box and weight <40lbs. This paper will overview the design and preliminary experimental results achieved in the early development of the engine. As a first step, a rugged "test rig" was designed for short term (uncooled) operation to validate the engine sealing performance, as well as the structural integrity of the engine in handling CI combustion pressures in excess of 100 bar. Several initial combustion chamber designs are explored, and matching to 1-D (GT-Power) model simulations is presented, for both indirect injection and direct injection type chambers. Preliminary data from firing the engine on Diesel (naturally aspirated) with a Compression ratio of 26:1 show the engine architecture functions well at demonstrated cylinder pressures up to 137 bar, and that the seals are functioning well. Future versions of the engine will include proper cooling sufficient for steady state operation and mechanical robustness to support long term development.