Unfortunately my options to help you in this situation are limited.
4 hours would result in quite a lot of shock to the corals, I would've preferred it to be as early as possible to make the change quick and unnoticeable to the corals as it can be.
The only thing left for me to suggest is to try and mix new saltwater after properly shaking the bucket, if uneven distribution of the salts was indeed the case it would result in slightly less concentrated Alk mix and would somewhat help to lower it.
Test the new water mix and post the results here before doing the WC.
4 hours would result in quite a lot of shock to the corals, I would've preferred it to be as early as possible to make the change quick and unnoticeable to the corals as it can be.
The only thing left for me to suggest is to try and mix new saltwater after properly shaking the bucket, if uneven distribution of the salts was indeed the case it would result in slightly less concentrated Alk mix and would somewhat help to lower it.
Test the new water mix and post the results here before doing the WC.