I have a 4 ohm Scout and have never tried the 8 ohm. A couple things to consider-
Many Walkabout owners, including myself, have used the Walkabout at 2 ohms on a regular basis without excess heat or issues. You can read many examples of this throughout the Walkabout Megathreads and other threads. Even Mesa says that it will work fine at 2 ohms, but they won't go as far as printing it in the manual.
The other side of the coin is that 300 watts can be alot of juice for one speaker, even by today's standards (most of the current made speakers that are designed to handle heavy bass and large wattage at the same time do so at the expense of the midrange that makes the Walkabout sound so awesome!). If you buy the 4 ohm cabinet, don't expect to be able to crank it while simultaneously cranking a lot of low end, or you will hit the limit of the speaker before you ever hit the power limit of the amp. Given the science behind it, the 8 ohm combo is probably a smarter choice given the single speaker- the volume difference shouldn't be much (3 dB), and you are far less likely to push the speaker to or beyond its limits!