Given:
- The ripple counter has 4 stages.
- Each flip-flop in the counter has a propagation delay of 20 ns.

To find:
- The maximum clock frequency in MHz.

Explanation:
A ripple counter is a type of digital circuit that counts in binary sequence. In a 4-stage ripple counter, there are four flip-flops connected in series, such that the output of each flip-flop serves as the clock input for the next flip-flop. The output of the last flip-flop is the most significant bit (MSB) of the counter.

The maximum clock frequency in a ripple counter is limited by the propagation delay of the flip-flops. The propagation delay is the time taken for the output of a flip-flop to respond to a change in the input. In this case, each flip-flop has a propagation delay of 20 ns.

Calculating the maximum clock frequency:
The maximum clock frequency can be calculated using the formula:

Maximum clock frequency = 1 / (2 * propagation delay)

In this case, the propagation delay is 20 ns, so:

Maximum clock frequency = 1 / (2 * 20 ns) = 1 / (40 ns) = 25 MHz

However, it's important to note that the ripple counter has four stages, and each stage introduces an additional propagation delay. Since the outputs of the flip-flops are connected in series, the total propagation delay of the counter is the sum of the individual propagation delays.

In this case, the total propagation delay is 4 * 20 ns = 80 ns.

Adjusting for the total propagation delay:
To calculate the maximum clock frequency, we need to adjust for the total propagation delay of the counter. The adjusted formula becomes:

Maximum clock frequency = 1 / (2 * total propagation delay)

Maximum clock frequency = 1 / (2 * 80 ns) = 1 / (160 ns) = 6.25 MHz

Therefore, the maximum clock frequency of the 4-stage ripple counter, utilizing flip-flops with a propagation delay of 20 ns, is approximately 6.25 MHz.

Round off:
Rounding off the answer to one decimal place, the maximum clock frequency is between 6.2 MHz and 6.3 MHz.