I've seen people on You Tube try many things and most don't seem to work. I've tried on my PS4 1200 series to replace the thermal paste with MX-4 and I also put a spacer between the top plastic shell and metal plate inside to try and put a little more pressure on the APU to make better contact with the heatsink but I did not hear much difference in fan speed or noise really.
I've seen some people on You Tube drilling holes or one large hole in the bottom of the PS4 to allow air directly into the fan which they say helps the most with reducing fan speed and noise. However, by doing this you also change the airflow within the console and potentially allow heat that was being sucked out to stay in the console and build up. The APU may end up cooler, but the rest of the console may not, so your taking a chance.
The Pro has the fan on the top of the console, so you would also see any holes you made or mesh you installed.
I've seen others use USB fans to either blow air into the sides, or suck air out of the back of the PS4. This is what I ended up doing. I have two AC Infinity 80mm dual ball bearing USB fans that turn on and off when the PS4 does and they suck air out of the rear of the console. They have an off, low, med, and high setting, in which I run on the high setting and the noise they make is on par with the PS4 fan when its just barely loud enough to be audible, so quite quiet. This helped make the PS4 fan run slower and more quiet, but after hours of gaming the heat would still build up inside the console and the PS4 fan would speed up and get noisy again.
I ended up going a step further and made an enclosure out of a 12 pack of mtn dew, lol, thats cut out and fits snugly against the rear of the PS4 to allow the hot air to enter it. Its even angled like the console and spray painted black to match so it doesn't look totally out of place. The back of the enclosure is cut out so all of the cables can easily be plugged into the rear of the console and the fans can be placed inside, sucking air from the rear of the PS4 and blowing it out the back of the enclosure. This has helped tremendously. The enclosure forces most of the air the USB fans are sucking to come out of the rear of the console and allows the PS4 fan to almost always run low enough that its inaudible. There is the rare time after hours of gameplay where BO3/DOOM/BF1 makes the PS4 fan slightly speed up for just a second but then it goes away. A very minor inconvenience overall.
In retrospect, I should have gone with 120mm fans, which weren't much more and could have been run on low or med and been completely silent. While the enclosure works great, it does hurt the super clean look of the PS4. I'd rather deal with a minor eye sore than a major ear sore myself. Not only is the stock PS4 fan loud at times, as it speeds up, the pitch I find is extremely irritating, and I'm not easily irritated by most things. That problem no longer exists now however.
Lets just hope the next PS console has better airflow and is more quiet. A slightly bigger shell than the Pro has wouldn't hurt.
PS1 - ! - We must build a console that can alert our enemies.
PS2 - @- We must build a console that offers online living room gaming.
PS3 - #- We must build a console that’s powerful, social, costs and does everything.
PS4 - $- We must build a console that’s affordable, charges for services, and pumps out exclusives.
PRO -%-We must build a console that's VR ready, checkerboard upscales, and sells but a fraction of the money printer.
PS5 - ^ -We must build a console that’s a generational cross product, with RT lighting, and price hiking.
PRO -&- We must build a console that Super Res upscales and continues the cost increases.
