With four 16 Ohm speakers, you can have two series-connected pairs which are then connected to each other in parallel to give a net impedance of 16 Ohms. Or you can connect all four of them in parallel to get a net impedance of 4 Ohms. If you connect all of them in series, the net impedance will be 64 Ohms - not very usefull. You could connecct two 16 Ohm speakers in parallel for 8 Ohms, but then you'd only be using two of the four speakers.

I explained the calculations in this thread if you'd like to see how it's done. The bottom line: You can't get an 8 Ohm impedance by connecting four 16 Ohm speakers together. And driving a tube amp into a load that doesn't match the output transformer is guaranteed to kill the amp.