You should only be doing pat/shush until she calms down, and you shouldn't be using it for weeks on end. Don't continue until she falls asleep because, you're right, it can definitely become a prop!
If she responds to pat/shush after 4 months, feel free to continue doing so (again, unless it's become a prop). Pat/shush worked for my ds until 5 months much better than PU/PD (which would overstimulate him at the time) so I used it when I needed.
Every baby is different and the month markers are rough guidelines. For example, do you continue until the beginning of the 3-month mark or the end? Do the babies changes one the first day of their 4-month bday? No... so it's a rough estimate.