Dielectric constant dictates the effective speed of light(RF) in the medium. Higher values make the antenna look smaller electrically. High performance RF boards will have a very tightly controlled Dielectric Constant and can provide detailed data for this over a range of frequencies. Cheap FR4 is likely to be much less controlled and vary from batch to batch and across frequency. In fact you may be lucky if a manufacturer will provide any figure at all for this, and you could be forced to guess at generic value.