Can I use Matzo ball batter after 4 days in the fridge?

Based on a couple of sources, prepared but uncooked eggs or dishes with them (this inclues prepared matzoh ball batter) are safe for 3-4 or 2-4 days' storage in the fridge.
So you're right on the edge.
If everyone in the house is healthy, go ahead and poach them.
If they're into the fifth day and/or someone there is either pregnant or immune-compromised, I would forego them.