loss tangent
The most important point with FR4 RF designs is, you have to keep a specific supplier and to assure additionally, that you are using actually the same quality. That's not obvious because major manufacturers have global production resources and may supply rather different products with the same name.
As said, GHz parameters aren't specified with FR4, even the basic parameter Er only roughly. Thus you have to evaluate the empirical parameters of your so-called FR4. If you don't like this way, use an explicite RF substrate.