Design a 4-bit ripple carry adder: four full adders chained so that the carry out of each bit feeds the carry in of the next. Inputs: a and b, 4-bit operands; cin, the carry in. Outputs: sum, the 4-bit sum; cout, the carry out of the most significant bit. The testbench applies all 512 combinations of a, b and cin and compares {cout, sum} with a + b + cin.