The recent study of Chernokulsky & Sikhov (2018) specifically cites A.I. Snitkovsky 1987 'Tornadoes in the USSR' as the source for the F4 rating. The source I've seen for the F5 rating is from the European Severe Weather Database, which has been changed subsequently to an F4 rating citing the 1987 source.
Now Wikipedia cites
this article hosted on a Ukrainian site, which appears that it may indeed be Snitkovsky 1987. I have a suspicion it may be only part of the original article which is supposed to be 13 pages, as Chernokulsky & Sikhov give specific ratings for each of the tornadoes citing that source, none are listed there, there is only a note that damage indicated that winds exceeded 100 m/s (i.e. F4 under the assumption of the time).
I think it is quite likely that the more recent Russian writing is drawing off later sources, possibly even English language ones (maybe even Wikipedia itself) citing the ESWD rating.