I've hit the 3-app limit on the free plan. How do I add a fourth app?

Two options. Upgrade to Pro (monthly, yearly, or the one-time Lifetime) and the limit lifts to unlimited apps. Or stay free and swap, remove an app you're not really using and add the new one in its place, keeping within your 3 free slots. Free never expires, so swapping costs nothing if three is usually enough.