import random

def generate_mnemonic_password(wordlist, num_words=4, separator='-', length=None):
    """
    Generate a mnemonic-based password from a list of words.

    :param wordlist: List of words to use for generating the password.
    :param num_words: Number of words to include in the password.
    :param separator: Separator between words in the password.
    :param length: Optional length constraint; will truncate or pad the password if specified.
    :return: Generated mnemonic password as a string.
    """
    if num_words > len(wordlist):
        raise ValueError("Number of words requested exceeds the size of the wordlist.")

    # Select random words from the list
    selected_words = random.sample(wordlist, num_words)

    # Join words with separator
    password = separator.join(selected_words)

    # Optionally truncate or pad the password to meet length constraints
    if length:
        if len(password) > length:
            password = password[:length]
        elif len(password) < length:
            password = password.ljust(length, separator)

    return password
