4 bikes? Even with the extended lever arm of the adapter, you are good to go if all you’re loading are bikes. A 1 1/4" class II receiver hitch should be rated for a tongue weight of 350 lb. So, if you can carry the rack without busting a gut, that’s 50 lbs. That leaves 300 lbs for 4 bikes. I think that you’re OK unless you’re hauling four battery assist bikes from a city rental rack. In DC, 70 pounds each! https://www.washingtonpost.com/...m_term=.3b2c0e83dcc4 (from DC Rainmaker’s blog here: https://www.dcrainmaker.com/...w-nov-26th-2017.html). And even then, you are just at the limit…and these things are probably overdesigned by a factor of 2 (that’s the engineer in me talking). Ride on.
It’s not just weight - as you say, weight ought to be ok. The Prius is designed to be very aerodynamic, of course, which means that a lot of air hits the bikes on the tray-style rack behind it. A bike (especially an aero bike) sideways is quite a parachute, and even a two-bike rack on the back of my Prius was a white-knuckle affair driving from Austin to Lubbock at Texas speeds (80mph limit) with the prairie wind. A similarly-light but much less-aero vehicle (Kia Soul?) would probably be a lot more pleasant for this task, let alone a bigger and heavier vehicle. A four-bike rack hanging off the back of a Prius would be like a giant slapping the rear of your car back and forth at random when the wind gusts, or a truck drives by.