Effects of 1, 2, and 3 wt% Si additions on the microstructure and mechanical properties of a cast Mg–4%Zn alloy was investigated. High temperature shear punch test was used for evaluating mechanical properties of the alloys in the temperature range of 25–250 °C. The obtained results showed that addition of Si resulted in the formation of the thermally stable Mg2Si intermetallic particles, which significantly increased high-temperature strength of the base alloy. The ductility, however, decreased after Si addition, mainly due to the presence of coarse Mg2Si particles having a Chinese script morphology. These particles can act as suitable sites for the initiation of microcracks, which leads to void formation at the interface of the Mg2Si intermetallic particles and the Mg matrix. To avoid ductility losses and to have strength enhancements, the Si content of the alloy should not exceed 1 wt%.