I drink 2 to 4 liters of (caffeine-free) diet soda each day as my primary/sole source of liquid in take. Is this a dangerous or unhealthy choice?

See previous answer. It could be a healthy choice depending on what your other choices might be. Drinking beer? Much better. Drinking water? Not so much. Drinking sugar sweetened soda? Way too many calories in 2 liters of soda. I would consider this amount of soda, juice, coffee or milk excessive. Moderation in all things.