If you have a 4ch plane and really want to learn rudder control then you could setup a switch to turn the ailerons off, sure. Then flip it on if you get in trouble or an attitude where the rudder isn't enough to correct it and flip the aileron back "on".

But it for sure always use rudder on the rudder stick. I have to clarify that because a couple new pilots I've taught, after suggesting only useing rudder for a time, just unplugged the ailerons and plugged in the rudder on the aileron channel. This really doesn't teach anything other than the plane can be manipulated by rudder. It didn't teach them to use their left thumb.