If I understand correctly what you want to do, this will work:
D = load('AI_Aorta.mat');
x = D.x;
idx = hankel(1:4, 4:numel(x)); % Index Matrix
xm = mean(x(idx)); % Mean Of Columns Of Resulting Matrix
To understand what this code does and how it works:
Check_First_Five_Columns = x(idx(:,1:5)) % The 'xm' Variable Is The Mean Of The Columns Of This Matrix (Extended To 302 Columns)
producing:
Check_First_Five_Columns =
0.1247 0.2370 0.1695 0.2010 0.2228
0.2370 0.1695 0.2010 0.2228 0.1482
0.1695 0.2010 0.2228 0.1482 0.1683
0.2010 0.2228 0.1482 0.1683 0.2063
