4.25
Enemies will also become a little harder in numbers not shown here too. For example, on higher World Tiers, challenging enemies will spend more time frenzied and thus will not be affected by many crowd control or stun abilities.
As you can see, while World Tiers increase the level of enemies you will go up against, they also dictate the level and rarity of loot you get.
On World Tier 1 you will see a lot of Greens and Blues dropping from enemies and chests in the world, however as you approach World Tier 15, you will be getting almost exclusively purple and even Legendary gear.