Thank you. Knee will tolerate it short term but i am more concerned about long term, with repeated application. It is very similar motion to what is done by folks sitting in "full lotus" w/ feet on tops of thighs, legs crossed. If done properly, all the rotational forces are supposed to be in the ball and socket hip joint. But I know of at least 2 people who thought they were doing just fine with it until one day stepping off a curb or some such, their knee blew out sideways necessitating major surgical rebuild. So maybe I will pass thank you, just to be sure I am still up and about in my 80's and hopefully well beyond that!
It's also possible to modify this stretch by having the leverage point of the stretch above rather than below the knee. In other words, you can be standing and fold the "4" leg but have it against a low counter or high bench touching above the knee, thus no direct pressure on the knee joint.
Another even easier way is to avoid the 4 and have the knees one over the other, so there is no space between the thighs, and pull the knees in toward the chest. Deep stretch in hip and back, no fuss muss or bother on the knee.