[2021] Horde night was too easy, how should I tweak it

RapFa makes a good point. Most of us are used to having the first Blood Moon on day 7, and it's often not very challenging for experienced players. But you're having one on day 4, so you've had even less time to level up and increase your days survived, which is what raises your gamestage that scales up the horde's difficulty. I would give it a couple in-game weeks and see how it's scaling up before making changes.

But if you do end up wanting to make changes, I would suggest zombie block damage as one way to make it harder without increasing the time to kill each zombie.