What is the impedance when I combine two eight ohm cabinets?
4 ohms

What is the impedance when I combine two four ohm cabinets?
2 ohms

Can I combine cabinets of different impedances?
It can be done, but it will cause a mismatch of levels. This is not recommended.

What is the impedance of an eight ohm cabinet combined with a four ohm cabinet?
2.7 ohms. This is not recommended.