the usual layer stackup for 4 layer is to bury the ground and power with the signal traces run on the outside.
Split planes are useful when you have multiple power rails but great care must be taken when crossing a split with a signal, lest you create a huge current loop and fail EMC testing.
Do use the hierarchical design features, they are there for good reason, and remember that 80% of the pcb is component placement and creating good footprints, get the placement right and the board usually almost routes itself, get it wrong and ratsnest does not begin to cover the horror.
Altium has really good design rules, use them (also net classes), most useful for cutting down on the potential for error.
Forget auto routing, it is almost always horrible unless you spend weeks setting up the rules.
Do remember to place the mounting holes (Embarassing, that), and if you have large modules that plug in (LCD?), then importing a step model is worthwhile as it can make catching mechanical issues in a compact assembly much less error prone, same thing if yoy have a complex housing.
Regards, Dan.