With regard to atama vs four dimensions, I believe four dimensions is more difficult. Four dimensions is pretty hard to follow already, and requires the most endurance of any ranked map out there. Atama, while technically difficult, can be practiced (mainly doing the squares sections over and over) until you have the muscle memory to pass them. You can just go into the editor and practice those sections individually 20 times over, until you can consistently hit them. You can't just go practice a rough section of four dimensions, because the map is just one giant test of stamina. I think in the long run, four dimensions turns out to be the harder map. Atama feels like it can be practiced and learned, whereas four dimensions is just a test of wills. Not many players are even physically capable of getting a full combo on four dimensions, much less a full combo with 96%+ accuracy, or even 98%+. Sure, maybe there are a select few streaming specialty players out there who can scrape through it with 88-90% acc and a decent combo, but that's not the same as throwing down a convincing replay, making the map look like it's your bitch. Only two players in osu can do that.
I give my nod to Four Dimensions for the current hardest ranked map.