Read the faq's and stickies to understand how bass gear works. A 4 ohm MINIMUM head can operate with any impedance that is not LESS than the minimum. So, 4 ohm head is good with 4, 8, 16, 32, etc. Less power is produced, but no harm.
Don't use the cab with 2 blown spkrs, as you don't know the impedance, and you *could* hurt your amp, and/or the other 2 spkrs.
The position of the volume knob is not an indicator of how much power you're using. Everything from your bass, any pedals, and all the eq knobs on the amp, all play a part. There's no way to determine how hard you're pushing an amp by the dial position.
Hope some of this helps, but like I said, spend some time with the stickies and faq's. Lots of good info there. Good luck.