To elaborate on the above post:
-Shengshou v3 is the best route if you are looking to get a good, cheap 4x4. The small size is nice because it makes the cube easier to hold/turn. However, it is slightly locky and doesn't cut corners that well (half a piece). That being said however, it still turns very fast and never pops. Plus, it is amazing out of the box, you don't have to do anything to it to make it good. It is my main 4x4 (over the Dayan AND X-Cube).
-The Dayan+mf8 4x4 is the best route to take if you do not pop cubes very often (you aren't a rough turner). It turns well, doesn't lock up, and cuts corners better than the SS v3, however, like I said, it pops quite a bit and requires a fair amount of breaking in/tensioning out of the box. A word of warning though, do NOT get the mini version. The full size version is larger than most 4x4s, but in the long run is quite a bit better than the mini.
-The X-Cube 4, in my opinion, isn't worth how much money it costs. They run anywhere from $40-$50, and require inner-layer modding/quite a bit of breaking in to make them great. Once that is accomplished, however, it is an amazing 4x4. It cuts corners better than the Dayan/SS and doesn't pop. In addition, it doesn't lock up. Overall, if you are willing to spend the money/time to break it in and mod it, you will be happy. If not, then this cube isn't for you.
TL;DR - Get the SS v3. Cheapest and the best out of the box.