One thing (among others) that I really like about the K4 is that you can flip the atomizer upside down and unscrew the coil assembly and replace the wick without removing any other parts. That makes me replace the wick more often when I do a juice fill.
The coil sits right above the top of the assembly. You can remove the wick with the chimney on and install a new one and screw the assembly back onto the bottom of the tank. While you have it apart you can fill the tank from the bottom with an 18ga needle on a bottle or syringe. The entire operation is quick and easy.
To get a K3 completely full, you have to use the juice port on the bottom of the atomizer. Taking the screw in and out is kind of a pain. Top fill leaves a large bubble, which is not a bad thing, but you can get more in with a bottom fill and still have a bubble large enough to allow proper juice feed.
Between the two, the K4 is a better choice, IMO. K3 is good, but the K4 edges it out.