andy, how do you define driver impedance of 4 Ohms?
It's the nominal impedance within the usage freq. range. For woofer, it is about up to 3KHz, after that the voice coil impedance kicks in to a gradual increase as freq. go up. The plot you have above probably comes from the impedance plot of a woofer. For tweeter, it is 4ohm almost flat up to 20KHz because obviously tweeter has to operate up to 20Khz. But you will have the xover which has its own impedance.
to get 4 Ohms total four driver spkr system I used 16 Ohms drivers, in parallel
It is very difficult to find a 16ohm drivers. Most are either 8ohm or 4ohm. The one you show there is probably 8ohm woofer.
parallel connected drivers have also direct connection to low output amplifier impedance, to dampen resonances.
Maybe for tube amplifiers but in which case the xover can use a zobel network to linearize the impedance curve. Remember it is not just the impedance of the drivers, it is also the impedance of the xover too.
For solid state amplifiers which has very high damping factor so the impedance of the drivers probably won't be an issue. So you won't have "dampen resonances" issues.
