For example,

import random

# The text to mangle
text = """Four score and seven years ago our fathers brought forth on this continent a new nation, conceived in liberty, and dedicated to the proposition that all men are created equal."""

# Loop over all the words in the text, split on whitespace
for word in text.split():
    # We don't want any punctuation after the word, so find the last
    # alpha character (A-Z, a-z) of the word, indexed at ilast
    ilast = len(word)
    while ilast > 3:
        ilast -= 1
        if word[ilast].isalpha():
            # We've found the last alpha character
            break
    else:
        # There's no point in doing anything with words with fewer than
        # four letters in them, so print them unchanged and move on
        print(word, end=' ')
        continue

    # Get the middle letters of the word as a list and shuffle them in place
    # NB strings are immutable so we can't shuffle word[1:ilast] directly
    middle_letters = list(word[1:ilast])
    random.shuffle(middle_letters)
    # Print the reassembled word, joining the initial, middle and last letters
    print(''.join(word[0] + ''.join(middle_letters) + word[ilast:]), end=' ')

An example output is:

Four srcoe and sveen years ago our fhtares bguohrt froth on tihs cnnnoetit a new naiotn, ccinevoed in lrtieby, and dtaecdied to the pposrotoiin that all men are creeatd euaql.