It wasn't so many years ago that the CAA were asking persons like myself (ATCO and authorised ROCC examiner for AGCS) to become FRTOL examiners too, now the wheel has come full circle.
As TheOddOne says, the use of either 4 or six digits when saying frequencies is driven by whether your country uses an 8.33khz spaced frequency. It is an ICAO requirement and at the time it was brought in, (I was on the UK Phraseology Working Group at the time) the UK had only one notified 8.33 frequency in use, but this meant we had to comply. We'd just about convinced the RAF to use abbreviated 'numbers' for 25 khz frequencies (eg 134.125 spoken as wun three four decimal wun too) when this happened so it was irksome to have to change back.
Interestingly, the USA didn't have to do the same as they didn't use any 8.33 freqs at that time!!(probably do now I would guess - unless someone here knows better)