A simple answer is "FR4 is G10 with more bromides in it".  This makes
the FR4 a flame retardent material which has a higher rating by UL and
other such designators.  Don't know if G10 is even around anymore or
can be used in many applications.
As an aside, when they started to add bromides in epoxy fiberglass (G10),
they never told people who had to plate thru the holes and this put a major
glitch in board manufacturers who wound up with voids as the process had
to change.  Many people will not remember this as it happened a long while
ago.

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