New fans ordered.
Thats pretty common with s4's at this point. The fans are starting to die on all the ones I had.
S4's are kinda dated at this point and most likely a lot have had multiple owners. I know I sold mine long ago and upgraded to S5's at the time. With them getting older having a extra fan on hand is a good idea. If multiple S4's maybe even a few, and make sure to clean them they can get very dirty over time. I like using compressed air personally, and if any dust on fan's wipe them off.
I owned a few a while back and only had to replace 1 fan. I used on of the sanyo's from the article they recommend, think I got it from Amazon at the time. But having extra parts is really part of mining to a certain point specifically if older gear.