The strength of a hook is often designated by the 2X, 3X, 4X, etc. icon. This is to tell us the thickness of the wire used when producing the hook, which can make a big difference on strength of a hook. For each "X" of strength, the wire used is typically used in a standard strength hook of each "X" up. 

In other words, a 2X Strong Size #4 hook uses the wire of a size  hook of the same hook shape in standard strength.

Keep in mind that numerous aspects of the hook can affect the strength of a hook, including dimensions, wire thickness, bend style, forging (or lack of), and wire material.