Which Layer should be considered for Characteristic impedance for 4 layer PCB?
A power plane should be an AC ground. It is if you've done it correctly, and it's not worth having if you haven't. A real ground is automatically an AC ground.

Reference outer layer signal tracks to the nearest AC ground layer. This configuration is called microstrip.

If a signal track is sandwiched between two AC ground layers, calculate the characteristic impedance with respect to both grounds. This configuration is called stripline, and has about half the impedance for the same width and ground spacings.