This is a really good topic as I never considered that a 4 wheeled walker could be less stable, in that it might roll when you might not want it to. The person using it would most likely have to have a good thought process to use the brakes on the handles, also.
I have a family member with Parkinson’s and wonder if a walker will be needed in the future. But if his thought process is also dulling at that point, a basic 2 wheeled walker seems it would be a better option. However, I do like the option of a seat on the walker.
I did find a 2 wheeled walker with a seat on Amazon but the reviews were pretty bad.