As a rule of thumb, after rebalancing, divide the total storage by the number of nodes. 54TB / 4 = 13.5TB per node
The new node has 74.5TB of capacity remaining, yet the total free space on all the other nodes is just 19.5TB. Therefore only 19.5TB of new objects can be added and the rest of the new node will not be usable.
1. Adding 2 nodes avoids complications and even avoids the need to do rebalancing. Can you add 2 nodes?
2. As a rule of thumb in small systems, if adding only one node it shouldn't be more than double the size of the existing nodes.
3. If adding 3+ nodes they can be any size (since they can regenerate to each other even if they are much larger than the existing nodes)