Returning Current on power planes? (High frequencies)
If you maintain low impedance between PWR plane and GND plane across frequencies of interest, either plane will work fine as a reference plane. That is how high speed boards are to be designed if you don't have a lot of time (man months) to simulate and verify the solution.

Remember: Fields can't read. So whatever label you put on some Cu next to a trace, that will be the reference and carry the majority of the return current.

With the type of circuit you describe, I will recommend the following.

Use a 6L board and do:

 Sig 
 ~100um prepreg
 Gnd 
 ~100 um laminate 
 Pwr 
 thick prepreg 
 Gnd 
 ~100 um laminate 
 Pwr 
 ~100 um laminate 
 Sig


This is provided you are okay with only 2 routing layers (avoid routing on L2/L5). And yes that may cost an additional 20-30% for the board - but you may easily save that in time spent (provided you value your time).

The two Prw/Gnd pairs builds some of the required high frequency bypass that you can't create with discrete bypass caps, which are good up to only 100 MHz or less (remember modern parts will have rise/fall times in the 2-300ps range meaning frequency content up in the GHz range - not sure where you get the 100 MHz from?).

If you make a ton of boards and have plenty of time, you can save a bit by going to a 4L board. But with a typical western world cost per hour it's not worth it unless you do at least 10K boards/year in my experience.

If you feel you absolutely have to save the money, the trick is to do a Gnd fill on the layer next to the Pwr plane and vice-verse. That builds up a high quality, high frequency Pwr/Gnd capacitor that you then supplement with discrete bypass caps. 

Use something like my pdntool.com tool to design that mix either way.

There are multiple reasons you want to maintain a low Pwr/Gnd impedance over frequency. EMC being just one of them. 

To sum it up:


You need low impedance between Pwr and Gnd for multiple reasons
From that follows that both Pwr and Gnd are equally good as return current reference planes


If any of this is not clear, please ask. I know I talk about 4 hours about this when I do courses, so it's a big subject. Very important however.

As to the other answers you have seen that suggest something else, the challenge is open: Show me a board that fails from following the reasoning I provided. Anytime.