FR4 is a class of printed circuit board base material made from a flame retardant epoxy resin and glass fabric composite. FR stands for flame retardant and meets the requirements of UL94V-0. FR4 has good adhesion to copper foil and has minimal water absorption, making it very suitable for standard applications. FR4 is either used as a copper-coated material for 1 and 2-layer PCBs or is used for multilayer PCBs as a Prepreg and core.
The standard values of the processed FR4 materials can be found in following PDF FR4 material - standard values.