So, is max speed up possible here. = 1/(1+1/s) = 1/(1+5) = 1/6 = 16.7%?

@shivalgo I believe that's correct (assuming we don't change the distribution of work). If we changed the distribution of work (evenly distributing the 1 + 1 + 1 + 2 units of work) to have each processor take (5 units) / (4 processes) = 1.25 units of work, then we now have a much less sequential part of our program, incrementing the maximum possible speedup.