Error 10251 - cannot fall outside the declared range

modulator is declared as an array of four six bit (ie, [5:0]) values, indexed as 0, 1, 2, 3 (ie, [3:0]).

Your default parameter value is asking for index 4'b0101 which is 5. 5 is greater than 3. QED.

So either change the default value to be in range 0..3, or in the module instantiation override the default parameter value.

OR if you really mean for the array index to be a four bit value then it should be declared as [15:0] instead:

wire  Modulator ;

since array selects are index values MIN to MAX (or MAX to MIN) and not address field widths.