I wouldn't use a for loop. MATLAB excells at array operations.
A = randn(12,5); % Make some data with positive and negative values
A(1:4:5) = -abs(A(1:4:5));
A(end-4:end) = -abs(A(end-4:end));
T = array2table(A)
rowIdx = find(T.A5 > 0);
rowIdx = rowIdx(:) + (0:3); % expands column + row
T(rowIdx,:) = [] % deletion
You can also only check every fourth row, but there's some tricky math.
rowIdx = find(T.A5(1:4:end) > 0) % check every 4th entry will result in the "block" index though
rowIdx = 4*(rowIdx-1)+1; % re-align to the right places in the original array
rowIdx = rowIdx(:) + (0:3); % expands column + row
T(rowIdx,:) = [] % deletion
