after I dropped my phone in water it started to have very poor Wi-Fi.

Hi @mbov777

Check if the LDI (liquid damage indicator) has activated indicating that liquid has entered the phone. - select Liquid or Corrosion evidence in the link.

If it has then the phone will need to be opened and inspected as to what damage has occurred to the internal components.

The impurities in the water causes corrosion and provides circuit paths for the electricity which were not in the phone's operating design and could damage the components. The corrosion starts immediately and is ongoing until it is completely cleaned away

First do not try to charge or to turn on your phone and remove the battery as soon as possible from the phone to minimize any further damage.

Then you need to dis-assemble the rest of the phone and clean all the affected parts using Isopropyl Alcohol 99%+ (available at electronics parts stores) to remove all traces of corrosion and water. If possible do not use "rubbing alcohol" as in some cases this is only 70% IPA or less, can contain additives which may leave conductive residues and is not as effective. If you do have to use it check the label to verify the amount of IPA. The higher the percentage of IPA the better

Here is a link that describes what happens. Electronics Water Damage

As always with electronics, especially surface mounted pcb be gentle when handling and especially when brushing away the corrosion. You do not want to remove any components from the board. Remove the shields that may be covering some components as the water may have got in under there. The ends of any flex cables and their connectors need to be cleaned as well

Hopefully after you have done all this the phone might possibly work correctly again.

Here is the ifixit Google Pixel 4 repair guide that may help with the disassembly.

If this process seems too daunting, take your phone to another more reputable, professional phone repair service, experienced in liquid damage repair and ask for a quote for a repair. If you decide to do this, do it sooner than later.