OK, you've got your Marshal head. But surely you must have seen people using a Marshal single stack (one speaker cabinet with four speakers in it) as well as the more popular Double Stack (two speaker cabinets with four speakers in each cabinet).
When you add a double stack to your head, the total is 4 ohms. Two 8 ohm single stacks in parallel.
The 4 ohms is not the ideal impedance rating, it is the minimum impedance rating. You can't go below 4 ohms, though likely an amp like a Marshal would tolerate it.
As far as wiring your speaker. You make two separate gangs of speakers of two speakers each, then you wire those two gangs together into the final speaker combination.
There are two way of doing this-
1.) Wire two speakers in parallel, then wire the other two speakers in parallel, then finally connect these two gangs of speakers together in series.
2.) Wire two speakers in SERIES, then the other two speakers in series, then finally connect these two gangs of speakers in parallel.
Either way the total is 8 ohms.
Two 8 ohm speaker wired in series = 16 ohms, but then you put two gangs of these in parallel, it drops down to 8 ohms total.
Two 8 ohm speakers wired in parallel = 4 ohms, but then you put two gangs in series, and the total becomes 8 ohms.
Steve/bluewizard