<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def is_visa(n):
 """Checks if credit card number fits the visa format."""

    n, length = str(n), len(str(n))

    if length >= 13 and length <= 16:
        if n[0] == '4':
            return True
    return False