Internal planes?
Look for the book 'high-speed digital design' by Howard Johnson, that's a good starting place. However, keep it simple for a 4-layer; use one of the internal planes as a GND plane (solid - don't split it), and then split your power plane. Normally, there are only a few devices which use 5V, so you can run a thick trace to those if they aren't critical... then you can use a small power island for 1.8V (assuming that some device is using that as it's core voltage), and leave the rest of the plane for 3.3V