If you haven't gained 4 life by the time an end step begins, Valkyrie Harbinger's ability won't trigger at all.

You create just one Angel token, no matter how much life you've gained past 4 life.

Valkyrie Harbinger's ability looks at how much life you've gained in the turn, even if it wasn't on the battlefield when you gained life. It doesn't care if you also lost life, even if you lost more life than you gained.