My fairphone 4 fell for a second in the toilet 3 days ago. It remained on for about 1 hour, before going out on its own. I then tried to turn it back on several times: it would turn on and off quickly. I then removed the battery and the Sim card. Then I read that it was not advisable to turn it on.
First, I would completely disassemble the phone. Fortunately this is quite easy with FP4. Then let it dry for at least 48 hours. Just as it is, no rice! If you’re lucky, it was just water and it will work again. If not, then it means some dirt managed to short some of the electrical circuits. In that case you will need to clean the main board (and possibly other components) with some isopropyl alcohol. And even then it could be too late and your last option would be to send it to Fairphone for repair.
Thank you for your answers. We have completely dismantled the phone and leave it in the open air for 48 to 72 hours before hoping that it will stay on again (at the last try, it would turn on and off immediately)