That's an electric heat pump water heater, so no, the "exhaust" won't be acidic. It will be cooler household air, with no combustion products to acidify it.

Running it out a chimney has several issues:

• potential to cause a lot of condensation in the chimney
• putting a 4" restriction on an 8" exhaust airflow (which is 1/4 the area, not half, if you slept through geometry.)
• forcing a chiney to work "backwards" (they naturally vent warmer air upwards - you'll be dumping colder air in at the bottom, trying to force it out the top.
• Possibly causing negative pressure in the house if you still have other gas appliances,
• and wasting the "free cooling" in summertime. In wintertime, it will draw in colder air from outside rather than just recirculating household air somewhat cooler for your heating system to reheat, causing additional load on your heating system.

This does not seem like a good plan.

That's an electric heat pump water heater, so no, the "exhaust" won't be acidic. It will be cooler household air, with no combustion products to acidify it.

Running it out a chimney has several issues:

• potential to cause a lot of condensation in the chimney
• putting a 4" restriction on an 8" exhaust airflow (which is 1/4 the area, not half, if you slept through geometry.)
• forcing a chimney to work "backwards" (they naturally vent warmer air upwards - you'll be dumping colder air in at the bottom, trying to force it out the top.
• Possibly causing negative pressure in the house if you still have other gas appliances,
• and wasting the "free cooling" in summertime. In wintertime, it will draw in colder air from outside rather than just recirculating household air somewhat cooler for your heating system to reheat, causing additional load on your heating system.

This does not seem like a good plan.