Saying which one is better, the Cube or D4 is not as easy as it sounds. I should know as I currently have both. Actually I have all three - the Cube, D4 and D4M.
To begin with, I ordered the D4 not as a replacement for the Cube rather than an alternative for when I'm hiking. The D4M came around only after I screwed up replacing the D4 head and knowing I could get the D4M faster than I could get the needed replacement part. So the D4M was a donor head.
I've used the Cube for several years and will continue to. The only drawback I have with the Cube is as a whole (tripod and head) while super sturdy, it weighs slightly more than 10 pounds. And since I'm getting older not younger the weight is beging to be too much for longer hikes.
The D4 is attached to a lightweight tripod and in total has saved me 5 pounds which is significant when hiking out beyond a mile or more.
From what I've seen so far, the Cube offers slightly better control of movements at the cost of weighing more. The D4 (now that I have it fixed) weighs less and has good movements. The movements of the D4 versus the Cube appear to be different but only slightly. The controls are also slightly different. All this is just a personal observation.
If pushed, I'd say the Cube is "slightly" better than the D4. However I've been using the Cube for many years and know it well. The D4 has only been with me for less than a month. I will say that both are great heads.
As for the D4M. I tried it out shortly before I operated on it. For a none geared head it works great. It isn't a ballhead so you can't expect the quick movements of one. But even without the geared portion you can still get precise movements like it's cousin the D4 - only slower.
The D4M is being sent to Precision Camera and is awaiting Arca to return from vacation.
My personal choice? The Cube is number 1. The D4 is so close to being number 1 that I have to change and say the Cube is 1a and the D4 is 1b. If you don't need geared movements nor a ballhead then the D4M is choice #2.
This is being written as I wake up and through my first cup of coffee.
Don