．、。
4. Right before it comes to a boil, lower the heat and skim off the scum one more time.
Literal
4. Come-to-boil-cause [just before-寸前で] weak-heat [into-に] doing, one more time scum [object-を] remove.
Step 4 of a recipe, in the standard instructional plain-form register. 煮え立たせる is the causative of 煮え立つ ('to come to a rolling boil') — the causative form is used here because you (the cook) are the agent causing the liquid to almost-boil. 寸前で ('just before,' 'on the verge of,' 'moment before') is a set phrase marking the point immediately preceding an event. 弱火にする is the standard collocation for 'lower to low heat,' using ~にする to indicate the change of state. 灰汁 (あく) is the foam/scum that rises to the top of a simmering pot — the skimming of which is a ubiquitous step in Japanese cooking.