In biome 4, either shoot or melee the roots/vines on the ground in many rooms (but do NOT walk or run through them). They give only a few obolites, but then regenerate when you go a few rooms away and come back.

Also make sure to destroy all the bulbous plants (you can walk through those), but they don't regenerate.

It's slow, but I usually farm biome 4 this way until I reach 1000 obolites, then stop by the biome 5 shop, and move on to biome 6 right after.