How do you like the box wedge?
I have no experience with a regular 4 way or 6 way, but a 6 way would be the most efficient as long as your rounds are not too big, because one stroke of the ram makes 6 pieces of firewood without the need for multiple strokes to feed a whole round through. A strong enough ram with a 12 way like Easton splitters have would be ideal at least in theory. With ideal wood, a 6 way should split up to 3 cords per hour, while the box wedge on my 19 ton splitter only goes through just over 1 cord per hour at best. Ideal wood and 30-40 tons may increase that to 2 cords, but I'm doing 2-4 ricks per hour normally with 1 or 2 hard working people.
The box wedge is good for regulating the size of the split wood to be on the smaller side no matter how big of a round you put through it. Of course too big of a round will send big chunks around the sides, which will need to be resplit. Other advantages include being able to split more difficult chunks of wood because you can manipulate it to split smaller and fewer pieces, and it doesn't split all at once because some of the wedges are 6" behind the others, so it requires less tonnage than a regular 6 way. Also, it uses the ram to pull out a crooked or jammed chunk of wood, but only the low pressure/high volume side of the pump feeds the return, which means it won't pull every jammed chunk of wood out.
Disadvantages include it weighing 175lbs, and the minimum height causes short or large diameter rounds of wood to tip over on their end when returning, making you have to wrestle the wood more.
Other considerations: keep the edges very sharp to cut through the wood fibers since it is impossible to perfectly line up the wood grain with all the wedges, you will need it to cut and extrude the firewood at times. Make sure the retract arm goes back far enough to fully pull a chunk off the top of the wedge, so you don't have to pull the chunk the rest of the way by hand. Also, make sure the corner in the arm is well braced to withstand repeated full pressure pulling forces. I assume the box wedge is messier than other wedges because quite often the remaining wood is not tall enough to split over the top or fit underneath, so it just shaves a mess off the last piece of a round to go through.