Set array of wires to integer value in Verilog test bench
Change wire to reg and you should be good to go.

And if you want to set them in binary, then you have to prefix with 4'b, like so: 4'b0000, 4'b1111, etc.  The 4 indicates the number of bits.  If you want decimal, then do 4'd0, 4'd15, etc.