[KathyC]

Aside from what has already been pointed out, you'd be taking a chance use a swimming pool liner as they are frequently treated with fungicides & algecides during manufacturing. They are not necessarily fish-safe.
A tarp may be useful as an underlayment for a liner to protect it from splinters, screw heads and sharp corners. Polyethylene tarps become quite brittle when used as a 'liner', IMO I'd doubt it would last 6 months without splitting.

Would this tank be for FW or SW fish?

The most appropriate liner would be 'pond liner'. It too is polyethylene and at a minimum it should be 20 or 30 mil thick. The better quality EPMD liners are 40 to 60 mil, but they are quite expensive.
Keep in mind that to line the pond you will have a lot of liner that will bunch up in your rectangular corners, so you would require a liner a minimum of 22 x 16 (or 18) foot for an 8 x 4 x 4 pond. You'd also have to be very neat in folding the material into the corners so that fish don't get caught in there, never mind the detritus that will accumulate. (This is where you'd use up the 'extra' yardage)

A preformed pond or a collapsible show tank might be a better way to go.
Best of luck with your project!