If the outputs of impulse are not scalor values, then store the output into a cell, where each cell can store a different size and type of data. Preallocating variables y and tl will speed things up and get rid of the warning message. https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html
y = cell(1, 4);
t1 = cell(1, 4);
for n = 1:4
[y{n}, t1{n}] = impulse(sys(n));
end