From what your describing it doesn't sound like a Grade 3 or 4 to me. Why would a Grade 4 be inoperable? I, too, would seek a second opinion. In the meantime I would discourage all jumping and buy stairs for the couch and bed. I would also no longer allow rough play with the larger dog. It's true that dogs hide pain, but with a Grade 3 luxating patella I would think you would see her walking funny at least from time to time.
I have some light weight plastic ones (the 4 step model - 16 " high) from
www.doggysteps.com (I bought mine off of Ebay though) and you can also get the 3 step model at Target, Walmart, Walgreens - haven't seen the 4 step at these places, but you can buy a kit to add the step from the
www.doggysteps.com - My dogs are all 4 pounds and they work great for me.