what 6x4 could mean

Indicates wheel setup..... Like a 4X2 is a two wheel drive pickup. A 4X4 is a four wheel drive pickup. A 6X4 is a four wheel drive, two wheel steer, three axle truck.
I suppose technically it would be eight wheel drive? But four drive wheel positions.An 8X8 would be a tandem axle truck with two front axles that drive also, thus eight driving wheels. An 8X4 would be your four axle truck again, with four driving wheels and two steer axles(not powered front axles).
I am not explaining this very well. There will be others on here that will do a much better job.

 

basically what the guy above said, truck with 6 wheel positions, four of which have power going to them.

 

Almost all 18 wheelers you will see on the road have 6X4 tractor drive configuration.
If only one axle powered (6X2, never heard of, but just for conversation), truck would be "stuck" every time on any uneven terrain.And in winter conditions (snow,ice,etc) truck would be stuck, most of the time.....
Occasionally see a tractor with one drive axle, and a drop axle, but not a common setup.

 

Dead axle tandems aren't that uncommon (that would be your 6x2).

 

Dead axle tandems sound like disaster in winter conditions? In the NE we mostly have tandems and tri axles (tandem with drop axle). A few Tridems around, but fairly rare around here....

Again, i am not very good at describing these things but here goes...
Most tractors in northeast area are 6X4 configuration. Power from transmission enters front rearend. Output driveshaft on back of front rear provides power to back rear. There is a differential in front rearend called a "power divider". This differential allows axles to differentiate, just like diffential in your car. Just as your car differential allows wheel with least amount of traction to spin, or allows outside wheel to turn faster in a corner, the power divider in a tandem allows one axle to turn at differnt speed.
The power divider can be locked out(like the locking differential, posi-traction in your car) to provide equal power and speed to both rears.
Power divider can only be used on poor traction conditions. Used on firm roads and the PD is not strong enough to take the abuse.
Could go on with many more details, but little steps. There is the power divider. Now they have axle locks so the four wheels turn as one, etc. More details at another time....
This is getting long winded so i will rest for now. Others will do a much better job explaining this.

 

6x4 is just a generic term meaning its a 3 axle truck with a twin screw tandem, not one with a tag. Most trucks I've ever drove, with the interaxle lock switch unlocked, one dual would spin. With the switch locked, 2 duals spin, whichever 2 have the least traction. I think you're thinking 6x4 means full locking rears, it doesn't necessarily.