Introduction:
To form four-digit numbers using the digits 5 and 7, we need to ensure that both digits are used an equal number of times. This means that each digit should appear twice in the four-digit number. Let's explore how we can achieve this.

Approach 1: Using 5 as the first digit
One possible approach is to use 5 as the first digit in the four-digit number. This means that 5 will appear as the thousands place and the hundreds place. We can then use 7 as the remaining two digits.

To illustrate this approach, let's consider the following steps:
1. Choose 5 as the first digit (thousands place).
2. Choose 5 as the second digit (hundreds place).
3. Choose 7 as the third digit (tens place).
4. Choose 7 as the fourth digit (ones place).

By following these steps, we obtain the four-digit number 5577.

Approach 2: Using 7 as the first digit
Alternatively, we can use 7 as the first digit in the four-digit number. This means that 7 will appear as the thousands place and the hundreds place. We can then use 5 as the remaining two digits.

To illustrate this approach, let's consider the following steps:
1. Choose 7 as the first digit (thousands place).
2. Choose 7 as the second digit (hundreds place).
3. Choose 5 as the third digit (tens place).
4. Choose 5 as the fourth digit (ones place).

Following these steps, we obtain the four-digit number 7755.

Summary:
By following the above approaches, we have formed two four-digit numbers using the digits 5 and 7, ensuring that both digits appear an equal number of times. The numbers are:
1. 5577
2. 7755

Conclusion:
In conclusion, to form four-digit numbers using the digits 5 and 7, we can either use 5 as the first two digits or 7 as the first two digits. By following the provided steps, we obtain the desired four-digit numbers.