If you have configured your PCB layers to have power planes and then decided you want greater flexibility with these layers they can be changed to normal signal layers in the Design Technology.
Once set to signal layers you have the complete flexibility of adding copper pour shapes, multiple copper pours assigned to the required nets and can add tracks etc. as required.
Launch the Design Technology (Shift + T) and select the "Layers Tab".
Two layers are defined as "Power Plane" in the "Bias Column"
Click on the "Power Pane" and the options are shown in the pull down list, set these as required, here "None" has been selected.
Next change the "Net" to "None".
The layer is now set as a normal signal layer. Repeat for the other Power Plane and the task is completed.
You now have a 4 signal layer PCB.
The names that you initially provided for the layers can also be selected and edited if they are no longer appropriate.