Hi Jeff,
You can try indexing as such
>> ALL.E.A % ALL is a structure with field E and field E has another field A
% To replace A field with 1/4 elements
>> ALL.E.A = rand(1,N/4); % Replace with the respective elements as you need
% Peform this for other fields B,C, and D
% TO replace the complete structure
>> ALL.E = F; % F is another structure
Hope this helps.
Regards,
Sriram