I disagree about the canter - I think the canter work in 7 is harder as it is on straight lines all the way around the school; the canter in 4 is (I think) easier for horses as it is mainly on a 20m circle at E/B. In 7 you pick up canter on the circle and then canter right the way around the school including both long sides - very easy for the horse to go crooked. Just my take.
Bu tin 4 you have to canter around the whole school and do a circle, though can see now where your coming from on the longside thing. My horse is not very good cantering circles though so 7 suits us more
P4 is quite alot of canter - the whole arena and a 20m circle.... some riders find it challenging if the horse tends to run on a bit as there isn't much room to come back to trot before the corner
I don't like 7, i find it way too boring and don't like cantering all the way round the school, and it doesn't suit mine as she is crooked (not that any judges have noticed it)
Not seen or ridden 4 so can't comment