Hi Jeanrose,
You could use a pulse generator block to produce a square wave of sufficient amplitude (usually 1, which is default), 50% duty cycle and time period as 1/(switching_frequency), which would act as a gating pulse for switches Q1, Q2 and correspondingly use a NOT gate for generating the gating pulses for Q3, Q4, as shown below:
I have attached the simulink model for your reference, you can tune the various block parameters as per your needs.
Hope that helps.
