You could do something like this
x = [-1 0 1 2 3 4 4 4 4 4]
n = 1:length(x)
y = NaN(1,n(end)) % preallocate providing NaN (Not a Number)for possibly unassigned values
idx = 4:n(end)-2; % valid range of indices
y(idx) = x(n(idx)+2) - x(n(idx)-3)
Note you have to take into account that x(n-3) is not defined until n =4 and x(n+2) is not defined after n=8
You could also pad the beginning and end with zeros and then only use the portion that you are interested in
