According to Ayatollah Sistani, the fourth intermediary no longer becomes Najes. The Najasa stops at the third intermediary. The example he gives is the following:
1- Your right hand touches urine, so it becomes Najes
2- Your left hand touches your moist right hand, so it becomes Najes.
3- Your left hand which is moist touches your clothes, and so your clothes become Najes.
But now the clothes don’t have the capacity to transfer the Najasa to something else. So if your clothes are wet, and they touch the furniture, then furniture does not become Najes though the clothes themselves are Najes.
In your example:
1- Your moist hand touches Najes leather, so the hand becomes Najes. Your hand here is considered the first Motanajjes, not the second, because the leather is the Najes itself.
2- Then your moist hand touches the doorknob, so it becomes Najes. The doorknob is the second Motanajjes here.
3- Then let’s say your moist clothes touch the doorknob. Your clothes also become Najes. But the najasa stops there. Your clothes won’t make anything else Najes.