Casting design is chosen by Soedarsono et al to maintain cooling rate in producing thin wall ductile iron (TWDI). Cooling rate should be maintained to prevent carbide formation. This paper discusses the effect of gating system design on microstructure and mechanical properties of 4 mm TWDI plate. A casting design based on vertical gating system is made to produce TWDI plates with the thickness of 1, 2, 3, 4, and 5 mm. This vertical system allows plates to function as runner which will helps in preventing premature solidification. Three designs were made. These designs are coded as T1, T2, and T3. These three designs were also used in making 1, 2, 3, and 5 mm TWDI plates of which the result has been published. Z-Cast is used to conduct a casting design simulation for filling flow and solidification. The result of flow simulation shows that the filling flow is resulted in two kinds. The result of solidification specifies that the 4 mm TWDI plates solidify in the third place. The result of the experiment highlights that in all of the designs, which have microstructure and consisted of nodule graphite in ferrite matrix, no trace of carbide and skin effect are formed. The length of skin effect varies in all of the designs. The highest nodularity is only 80% while nodule count is 931 nodules/mm2. Brinell hardness number for all of the design is beyond the standard given by JIG G5502. As for UTS, yield strength and elongation none of the designs exceeds the minimal standard. The result of the experiment does not confirm the result of the simulation. In sum, compared to the previous result, the curve trends of 4 mm TWDI plates look similar to 2 mm TWDI plates.