Manga Volume 4

 is the fourth volume of the manga adaptation of That Time I Got Reincarnated as a Slime.

Synopsis
A CALL TO BATTLE

A living disaster in the form of an Orc Lord and his 200,000-strong force of monsters threatens the residents of the Great Forest of Jura. In response, Treyni the dryad, protector of the forest, asks Rimuru to vanquish the Orc Lord and his army. Rimuru accepts, but to take on this new, formidable enemy, he'll need help. Unfortunately, it looks like he'll need to enlist the aid of the lizardmen, which might be difficult after his last encounter with one of their leaders...

Chapters

 * Chapter 18: Loosening Gears
 * Chapter 19: False Superiority
 * Chapter 20: The March to War
 * Chapter 21: Ripples on the Battlefield
 * Chapter 22: Death Storm

Bonus Short Story
 * Veldora's Slime Observation Journal: Pitched Battle