Working with plane layers (How to split a plane)
For planes that will be continuous, you can select a plane layer in the board configuration. From the menu bar select Design > Board Configuration. With a multilayer configuration selected check the layers you would like to use as planes then press OK.
To assign each plane layer to a net hover your mouse over a trace or rats nest line attached to a net you would like assigned to your plane layer. Right click and select properties.
Near the bottom of the Nets properties tab, you will see the inner layers listed and labeled "no nets assigned". Place a check mark in the box next to the inner layer you would like to assign to the selected net. Press OK to save your changes. Repeat this process for all internal plane layers.
If you would like to split the plane layer, you can do so. With the inner layer already assigned to a net, you can divide the layer using a polygon line. Any polygon features added to a plane layer will remove copper. You can assign multiple nets to the same plane layer, but then you must make sure that the nets are separated. In V4, you can select a net name from the PCB Object Hierarchy on the Project panel. The Project Panel is a tab located at the bottom of the Edit Panel. Once on the Project Panel select PCB Node => PCB Object Hierarchy => PCB board => Children to see a list of all of the objects on your layout.
Once you select the desired net name the entire net will be highlighted. This will allow you to see the connections to the net. You can use this to make sure the nets are separated and adjust your polygon lines accordingly.
An alternative method that will work is to use filled polygons. If you choose to use this method, you should uncheck plane layer selections in the board configuration. As I mentioned earlier, adding polygon features to a plane layer will remove copper. You can add two or more filled polygons to a layer and assign each to a net. Each filled polygon can only be assigned to one net. Once you have the polygon in place, press esc twice to return to the selection tool, then right click on the polygon and select properties. You can change the layer the polygon resides on, name the polygon (optional) and assign the polygon to a net. Now that the polygon has a net assigned, it will only connect to other pins/pad/traces of the same net.
You must make sure that two polygons of different nets are separated. A keep out will not be created if two filled polygons overlap.