R & S English is an outstanding program, and it is considered advanced.
That being said, what you have done before for grammar will make a difference in ds's testing as well, as one year of something can help dc make good gains, but it is not the the same as multiple years of instruction with R & S English.
HOD suggests trying to complete R & S English 6 by the end of Grade 8 (as R & S English 7 and 8 are high school credit worthy). So, I would say you have a few options...
Option 1:
Ds's fifth grade year: half of English 4
Ds's sixth grade year: other half of English 4
Ds's seventh grade year: English 5
Ds's eight grade year: English 6
Option 2:
Ds's fifth grade year: English 4
Ds's sixth grade year: half of English 5
Ds's seventh grade year: other half of English 5
Ds's eighth grade year: English 6
Option 3:
Ds's fifth grade year: English 4
Ds's sixth grade year: English 5
Ds's seventh grade year: half of English 6
Ds's eighth grade year: other half of English 6
I would say I'd do the option that is written in the HOD guide you are using, as this will give good balance to the day.
I'm not sure which guide you are using though, and there are several options within a guide. So, I'd then consider what pacing ds seems most well matched with.
I hope something here helps!
In Christ,
Julie