Can I replace one 250 lb propane tank with four 50 lb tanks in parallel. The tank supplies a hot water heater and occasionally a fireplace. I can save quite a bit by taking the smaller tanks in to refill myself as opposed to having the gas truck deliver.
You can split the system and probably don't need 4 tanks but at least 2. If you are renting the tank and regulator you may need to buy a new regulator. Some states require restraints for smaller tanks these can be as simple as a 4x4 post in the ground with a snap chain to each tank. The post also provides a secure mounting location for the regulator. Some state require tank straps attached to the support a bit more expensive.
Look at: TANK SIZING to see how size affect BTU/hrs
I would make sure what your two loads listed total BTU/hrs fit in the tanks supply ability. Smaller tanks in parallel need to add up to be able to supply the required BTUs/hr.
You also will need to provide the proper regulator to hook into your multi-tank manifold.
I haul two 100lb tanks for the same reason you list.
I run a stove, LP dryer and tankless water heater on the two, but am considering adding two more tanks as I am pushing the limits when all are on at same time.