High speed signal return path in a stack-up with multiple ground planes
In this design, the return current of high speed digital signals in layer 4 will be almost equally distributed between layers 3 and 5. This is OK until layer 3 has no discontinuities (isolation gaps) along any of digital lines. Careful routing may help to achieve this goal. Then your planes 1 and 2 stay "clean" of digital signals.

If you can not avoid gaps in layer 3 across the digital lines, you have to place them in Bottom layer.