zarezare wrote:I checked speakers and found that bottom one missing. Other three are in excellent condition, so now I have problem where to find the 4th speaker.
Difficult, if you want an exact match - the speakers are long out of production and almost unique to this cabinet (only shared with the Quad Reverb combo as far as I know)... unlike all other Fender speakers they are 16-ohm, because they are wired all-parallel to give a 4-ohm cab. The only place you're likely to find one is in another Bassman cab.
I wouldn't bother - they aren't very good for bass. They have low power handling, a lightweight 'seamed' cone and are not very sensitive. This means that you have to drive them quite hard to reach a good volume, and then they distort in a not very musical way at low frequencies, even if they don't blow entirely - which is probably what happened to the other one in your cab, and it was then removed to stop it sounding awful. They're OK for guitar, but I wouldn't choose them if you're actually going to use it as a bass amp.
I would get four decent modern higher-sensitivity 16-ohm drivers of at least 100W handling each, since the amp will peak at at least 200W when pushed hard. For bass you generally don't want to overdrive the speakers at all even if you're using amp distortion, so another factor of two margin is a good thing. Eminence do several speakers which fit this description as far as I know, which are probably at least in the same tone ballpark as the originals.
Or, just start with one and see if the other three speakers do actually sound OK, you have nothing to lose really.