Considering you mentioned the fans are going full throttle unlike before your cleaning attempt (?), the following can be the cause:
- You’ve damaged either the leads or the fan connector, causing poor PWM connection (the connection used to control fan speeds)
- Your re-assembly has degraded cooling performance to one or several components.
My advice is to ensure first and foremost you’ve not damaged the fan leads/connector in any way. This is the easiest and quickest diagnostic. If damaged, you can try repairing the damage (if you’re comfortable with that), or order a new fan.
If that doesn’t help, make sure all thermal pads are covering all chips that are intended to be cooled by them. Sometimes these dry out, gets misplaced, easily torn, or simply aren’t long enough to eg. cover all VRMs. VRMs are arguably the most important component to prevent overheating, as these deliver power directly to other components like the SoC (the “CPU”).
If I may ask, why did you re-build the PS4?