Some of my pets have reached level 10, and as such, now have 4 abilities listed when I view them in the 'pets and mounts' window. However, when I am in battle, I can only see three abilities. What am I missing?
You can only have three abilities active at any given time. When you look at your Battle Pet Slots in your Mounts and Pets window, there should be an arrow underneath your pet abilities. Clicking this arrow will allow you to select one of two abilities to assign to that slot.