Technically 6 ohms. But only for the frequencies where the 2 speakers intersect. In other words if your 6.5" is playing the frequencies 80-2000 hz and your 4" is playing 150-4000 hz, the amplifier will see an 8 ohm load for frequencies 80-150 hz, a 6 ohm load for frequencies 150-2000 hz and a 4 ohm load for 2000-4000 hz.
The final impedance of a speaker load will ALWAYS be less athan the lowest impedance in the formula.
It's a load if you take a load and put a lighter load on top of it then it does not get lighter.
But, as already mentioned this is only true if the drivers are playing in the same passband, and remember it's impedance not resistance. These are nominal numbers and impedance curves can widely vary from driver to driver.