The RC12L4 shown at the Snowbirds has two changes from the RC12L3. First, the chassis is a thicker plate with fewer cutouts than the 12L3 chassis. This looks even stiffer than the special thick plates made for the team drivers. Second, the castor blocks on the front suspension come in 5 degree as well as 10 degree and 0 degree reactive castor. There is also a place in the middle of the castor block to mount a retaining collar for the hinge pin or mount a stiffener that connects both sides. This appears to be their response to the IRS castor block system.