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FR4 is a composite material, in most cases its a good approximation to assume a dielectric constant Er of 4.2
The two main constituents of FR4 are glass with an Er of around 6 and resin with a dielectric constant of around 3. Typically manufacturers quote a dielectric constant for the composite "c" stage material of around 4.2.
The dielectric constant may vary, depending on manufacturer. Values between 4 and 5,6 are possible, but a good approximation is 4,6. Best thing is to check manufacturer datasheet. Another factor which may vary is loss factor, which is important in high speed. Also, dielectric constant is not really constant over a wide range of frequencies, but up to few hundreds of megaherts, this variation can be ignored.
If I remember correctly the FR stands for Fire Resistant. Not sure what the 4 stands for. This type of board is made of fibreglass as opposed to the cheaper phenolic board whic burns quite easily.
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