I don't do exams so I've never got anyway near grade 8.
I do have the grade 5 ABRSM scales book which shows arpeggios over three octaves grouped in 4. I also have a copy of Hanon which has them in four octaves grouped again in 4.
I'm not sure if I have interpreted your question correctly.
I think you have interpreted my question correctly.
I have a copy of the ABRSM grade eight scales book which does show them grouped in fours. Played 1234 1234 1234 1234 if I'm reading it correctly.
It's just that a student I know was asked to play them in fours by one teacher and in threes by another. As it happens the student can play either with ease but obviously would like a definitive answer prior to doing the exam.