If the file replacements.txt looks like

- word2:replword2
- word4:replword4

then you can build a dictionary from it pretty easily. Here's one way to do it.

replacement_words = {}
with open("replacements.txt") as file:
    for line in file:
        line = line.lstrip("- ").strip()
        word, replacement = line.split(":")
        replacement_words[word] = replacement

print(replacement_words)

my_string = "word1 word2 word3 word4"
print(my_string)

for word, replacement in replacement_words.items():
    my_string = my_string.replace(word, replacement)
print(my_string)

If your replacement file doesn't have to look like that, you could put it in a standardized form. For example, JSON.

{
    "word2": "replword2",
    "word4": "replword4"
}

Then the parsing part can be handled by a library.

from json import load

with open("replacements.json") as file:
    replacement_words = load(file)

print(replacement_words)
my_string = "word1 word2 word3 word4"
print(my_string)

for word, replacement in replacement_words.items():
    my_string = my_string.replace(word, replacement)
print(my_string)