What do the numbers 1, 2, 3, and 4 mean on the recycle triangle?

Answer 1

The triangle numbers identify types of plastic to facilitate sorting and reuse. Different plastics may require different chemicals to dissolve for reuse.

Answer 2

The number in the recycling triangle on plastic objects indicates the material it is made of. Usually there is also 2 to 4 letters underneath the triangle as well. These letters also indicate the material.
For instance 1 with the letters PETE means Polyethylene terephthalate. If you want to see them all look at the source link.