First, you can't assign the variable twice.
Second, your range is off, 2 bits can only go from 0 to 3. You need a 3 bit output to count up to 4.
This is more like what you need:
module ones(
output wire [2:0] one,
input wire [3:0] in
);
assign one = in[3]+in[2]+in[1]+in[0] ;
endmodule