Grade 6 theory is HARD(according to my teacher's marking in my trial exam)
Grade 4 is more simple.
Intervals-
1.All chords start with major except 4th, 5th, 8ve.(eg..Major 2nd, Major 3rd.....Perfect 4th...)
2.Major 2nd-2 semitones, Major 3rd-4, Perfect 4th-5, Perfect 5th-7, Major 6th-9, Major 7th-11,Perfect 8ve-12
Just memerize 2,[4*,5*],7,9,11,[13*]
*=perfect intervals
Minor=down 1 semitone(flatten)
Augmented=up 1 semitone(sharpen)
Diminished=down 2 semitones(flatten)
Intervals must always be remembered.(draw a keyboard on the manuscript paper to count semitones)
I hope this helps.