You want to have a wallet in which transactions can be made if they are signed by person A and one of B, C and D. Is this what you are trying to achieve?
If so, you can create a 4 of 6 multi-signature wallet in which the keys are A1, A2, A3, B, C and D. (A1, A2 and A3 are all owned by one person.)
Can anyone explain in an accessible way why split A into A1, A2, A3, if, as a result, all these 3 keys will belong to one person?
With A1, A2 and A3 being with just one person, that is he will have to back 3 keys and 3 Seeds.
All 3 keys can be derived from the same seed.
Especially if A1, A2, A3 can be obtained from one key.
Why complicate? Can't this be simplified?