Verilog code to implement a 4-bit full adder circuit.

Here’s an example Verilog code to implement a 4-bit full adder circuit:

module full_adder(
  input A,
  input B,
  input C_in,
  output S,
  output C_out
);

  wire sum1, sum2, carry1, carry2;
  
  // First stage
  xor(sum1, A, B);
  and(carry1, A, B);
  
  // Second stage
  xor(S, sum1, C_in);
  and(sum2, sum1, C_in);
  or(carry2, carry1, sum2);
  
  assign C_out = carry2;
  
endmodule

In this Verilog code, the `full_adder` module takes three 1-bit inputs `A`, `B`, and `C_in`, and outputs two 1-bit signals `S` and `C_out`. The `S` output is the sum of `A`, `B`, and `C_in`, and `C_out` is the carry output.

The full adder is implemented using two stages. The first stage generates a partial sum and carry output using XOR and AND gates. The second stage generates the final sum and carry output using XOR, AND, and OR gates.

The `xor`, `and`, and `or` operators are used to implement the logic gates in Verilog. The `assign` statement is used to assign the value of `C_out` to the output of the second stage.

This is just one example implementation of a 4-bit full adder circuit in Verilog. A 4-bit ripple carry adder can be implemented using multiple instances of the full adder module connected in series.