Start by determing the real electrical (and mechanical) parameters of the speaker. 4Ohm 50W tell you absolutely nothing of value.
Are you trying to make a vortex ring device using a speaker? I'm working on that, too. I'm not going to type in loads of stuff until I find out what it is you are trying to do.
I'm trying to make a different way to give tactile feedback.
analogWrite()DescriptionWrites an analog value (PWM wave) to a pin. Can be used to light a LED at varying brightnesses or drive a motor at various speeds. After a call to analogWrite(), the pin will generate a steady square wave of the specified duty cycle until the next call to analogWrite() (or a call to digitalRead() or digitalWrite() on the same pin). The frequency of the PWM signal on most pins is approximately 490 Hz. On the Uno and similar boards, pins 5 and 6 have a frequency of approximately 980 Hz. Pins 3 and 11 on the Leonardo also run at 980 Hz.