First of all, it's "herblore" not herblaw, and to make a potion into a (4) dose, you just need to use one on the other. So say you've got 4 (3) dose prayer potions. If you use one (3) dose on another, you get a (2) dose and a (4) dose. You can also take your (3) dose potions to the herblore guy next to the GE and get him to "decant" your potions for you.
This won't get you any herblore xp by the way.
¬Albal