As with all other abilities, there are two choices for Pick Up Block's level IV upgrade.
Blocky Boomerang: Block comes back after successful mob hit
Blocky Boomerang lets you re-use one block many times as long as you hit a mob with the block. Is really cheap because you don't have to pick up any more blocks so it's a chi-free attack after the first one. This upgrade is recommended very skilled earthbenders.
Large Impact: The block has a 1/2 chance to create an explosion on landing
This upgrade allows the picked up block to (with a 50% chance) explode upon hitting an entity or a surface. The explosion can be deadly to the majority of the mobs but it doesn't destroy terrain. Because of the explosion, this ability becomes really good against hordes of enemies.
Sometimes the explosion destroys the drops from mobs.