I was wondering if a 4 mil liner would keep the water in or if it would need to be stacked in layers.
4 mil is the same as a trash bag. It will not work because it isn't water tight. Doubling or tripling won't work either. The water permeates one layer, then the next and so forth. And pretty soon the water is on the floor and the plastic bag is floating on top. (Guess how I know this?
If you want an indoor pond, use a plastic pot or a horse trough from the feed store or a large muck bucket depending on what size you want.