Well, your total impedance between the two cabs is 2.67 ohms.
The power will be split between the cabs based on their impedance. The 4 ohm cab will be receiving twice the power of the 8 ohm cab. So at whatever power you are operating the amp at, 2/3 will go to the 4 ohm, 1/3 will go to the 8 ohm.
Although I wouldn't run any concert at less than 4 ohms doesn't mean that you can't.
There is no disadvantage other than the potential for the 4 ohm cab to be much louder than the 8 ohm cab - that is dependent on the efficiency of your speakers. But if they sound balanced in output enough to you, then it isn't a disadvantage.