The function replaces "a" at the starting of a string with "z", "o" with "V", "e" with "V", and "r" with "d" in a given input string.

[CODE]:
def modified_string(s: str) -> str:
    s = list(s)
    if len(s) > 0 and s[0] == "a":
        s[0] = "z"
    for i, char in enumerate(s):
        if char == "o":
            s[i] = "V"
        elif char == "e":
            s[i] = "V"
        elif char == "r":
            s[i] = "d"

    return ''.join(s)