This is a sequential circuit. Read the simulation waveforms to determine what the circuit does, then implement it.
2030405060708090100110120130140150160170180190200210clkaq4560123456014560
module top_module (
input clk,
input a,
output [3:0] q );
always@(posedge clk)begin
if(a)
q <= 'd4;
else if(q == 6)
q <= 4'b0;
else
q <= q + 1;
end
endmodule