After the last round is played do you still discard down to 4 water tokens, or can you score more than 4 in the final scoring?
Actually, the rules guide you to discard even before the round has ended. This quote with my highlight.
Discard water well tokens
After a player has completed his second sales cycle, he checks the number of his water well tokens. If he has more than 4 tokens in his personal supply, he must
discard any surplus tokens of his choice, coin side up.
From the section that immediately follows:
End of a game round
The game round ends after all players have completed their second sales cycle or passed.