The 4XL by Australian manufacturer Werrd is a smooth concave shape, slightly undersized and very comfortable in the hand. The name 4XL comes from it's unique use of 4 (yes FOUR) bearings - 2 recessed size-C bearings in a "beefcake" configuration give the 4XL a slight center groove. On the outside, the 4XL uses YoYoFactory licensed Hubstack Technology for it's "Jimmy Hat" stacks - they are somewhere between a standard small hubstack and the large Z-Stacks used by YoYoFactory.
Shown here is a colorway called "The Works", which is Red and Black with Silver acid wash. It was made as part of an ill-fated charity drive by Vs. Newton Y-Yo Concepts.
