How B Tree guarantees self balancing property?

Let’s say we have a b tree of order 4 and it has 3 levels. All nodes are completely filled. Now if we want to insert a key with maximum value ( than all keys present in the tree). For this, we have to go to the rightmost node at the last level. As this last level is also filled, now how can we proceed further as all the nodes are full. We cant do a split here. So we are left with creating a new node below the rightmost node. But this tree contradicts the balancing property. I want to know the intuition behind self-balancing nature.