GCRFGV04 is not a valid GC-code (yet) and it probably won't be for some time.
In short: They are assigned consecutively. In order for the given GC code to be valid, the consecutive numbering/naming nees to reach 8 characters (in total - 6 if you discount "GC" at the beginning) and go into that namespace for a bit.
It has taken since the inception of Geocaching in May of 2000 to reach where we are now. Going through 1-4 characters after "GC" took from May of 2000 to December of 2006, all of which was when Geocaching was in its infancy. Since December of 2006, we have been using 5 characters after "GC".
There are 31 characters used for GC codes, including all 10 digits from 0-9, but excluding certain normally present English letters because of potential confusion with numbers when hand-written (O for 0, 5 for S, 1 for I, etc.). New caches today begin with "GC9" followed by 4 more characters. When "9" runs out, we get "A" (like in "GCAxxxx"). When that happens, there have been a total (theoretical) max of 28629151 caches (31^5) with 5 digits after "GC" created. In order to reach "GCRFGV04", you'd need to enter the 6-character realm, each of which will have the same amount of combinations (28,629,151). "R" would be the 24th of those, bringing it to (31^5)*23 = 658,470,473 to reach GCR00000 - from which point (by the way), you'd still need about 1.5 as many caches as has ever been created to date to go from GCR00000 to GCRF0000.
It is not that the GC code is invalid per se - it is just that it will probably take a loong while for someone to create it :)