I have the identical splitter purchased this fall. I was able to con NT out of a set of 4-ways for some trouble I had (another story). There are many people who claim the only way a 4-way should be used, is on a wood splitter with the wedge "FIXED" on the end of the I-beam. Not on splitters with the wedge on the piston end (like yours). The reason being is the 4-ways like to twist when hitting knots or hard spots, which isnt good for the cylinder. Some feel the 4-way actually needs to have the freedom to ride up and down during the process.
What I have found with my set is when the 4-way wedge starts splitting, the bottom 1/2 of the log under the 4-way is sandwiched between the wedge and the I-beam. So there is much added pressure on the splitter to try and push the wedge along the sandwiched piece of wood to the end of the stroke. The top 1/2 of the log is free to split away without resistance, but the 4-way is riding the bottom 1/2 the whole time and the wood has nowhere to go. For splitters with the 4-way attached to the "end" of the I-beam, the resistance isnt there. The wood pretty much just falls off the end of the splitter.
One more note. You almost have to be satisfied with the size splits you get when using the wedge. It seems like some are too big, which is fine if you dont mind. But you can't simply take that large split and just split it again, otherwise you will quarter it up into 4 splits which are too small. If you had the perfect sized wood all of the time ( 16-18"), the splits would be fine. But thats not my case. And I dont want to take the wedges off and re-split a pile I just got done splitting. But this could be done with a ax.
I know many have 4-ways, and love them. There is a 50-50 chance I will use mine again someday. But to me, it's not worth the $70, maybe if you run across a deal, it may be worth a try. I found them to be hard on my splitter, so I hate to take a chance on ruining something that can be avoided. One thing nice about owning a splitter is we have the option to split at our leisure. Much nicer than renting one for a weekend and "having to bust your nuts" until the job is done. So if it takes a bit extra time without a 4 way, I'm willing to take the time.