This document describes a 4-bit parity checker circuit that uses XOR gates to calculate the parity of 4 input bits (a0, a1, a2, a3) and output the result as a single parity bit (p). It defines the circuit entity with 4 inputs and 1 output, then the architecture uses 3 signals (r, s, p) to sequentially XOR pairs of inputs and pass the result to calculate the final parity bit output.