P-Tex is rated by how many pores it has in a certain area. A 4000 base would have approx. 4000 pores, a 2000 base 2000 pores, etc. The more pores the base has, the softer it is, but the more wax it soaks up. Thus, race bases which have a lot of pores to soak up tons of wax are generally softer and more fragile. Manufacturers do add minerals to bases to make them harder- for example, many Salomon bases are made of Galium 4000.
When it comes down to it though, the rock that will give you a coreshot on a 4000 base will most likely give you a coreshot on a 2000 base as well.