It’s December 5th instead of December 4th because 2020 is a leap year on the Julian (!) calendar. For more information: Chabad.org: Why Is the Prayer for Rain Based on the Civil Calendar?
Bonus yekke joke. A yekke comes home from shul one evening. His wife jumps up and says, “Gottfried! You are so late! I thought you died in a traffic accident!” The yekke says, “Ach, Hilde, you’re so silly! You forgot that today is December 5th! We were saying ‘tal umatar livracha’.”