close up filter queries

[leowyien]

What does +4 on a close up filter mean?

 

[Numnumball]

Its referring to diopter size in terms of magnifying

Technically speaking, it's a term used to identify the refractive (light bending) capacity of a lens. In close up filters, there is a direct correlation between focal length and diopter. Each diopter increases the size of the viewed object by 1/4 (25%) when the object is at its full focal length from the lens.

Hope this clarifies