Best 4-layer stack-up for a board with both analog and digital circuitry?

Signal/ground/power/signal is the right starting stack-up, no notes there. The split ground habit, though — that's the one piece of received wisdom from a decade-plus ago that's actually aged badly for most boards, including yours.

Current best practice is a single, solid, unbroken ground plane, with the analog/digital separation handled by component placement and routing discipline instead of a physical split. Reason: the moment any signal needs to cross from the digital side to the analog side (and on a board with an ADC, something always does), that signal's return current either has to detour around the split — adding loop inductance and often radiating more, not less — or you end up needing a stitching capacitor across the split anyway, which mostly defeats the point of splitting it in the first place.

What actually works: keep the ADC and analog front end physically grouped on one side of the board, route the digital bus and clock lines away from that area, and let the solid ground plane do its job underneath both sections. One thing that is worth doing deliberately: route the connection between analog and digital sections (your ADC interface) through a single, narrow corridor rather than letting digital traces sprawl close to the analog section from multiple directions. See our PCB power and ground plane design guide for the full reasoning on why solid planes generally beat splits.