4 Layer PCB Standard Stackup Grounding
Ground pour is a generally a good starter move, nice way to separate potentially interfering signals, but not necessary. Make sure you stitch to your main reference plane with vias. Consult with your board house, but it can help with very high aspect ratio traces which can overetch. Don't waste a whole layer with the 5 V pour, it's not really necessary. Certainly layout your power distribution there, but don't use that for a full, non-interrupted pour.

EAGLE will definitely do vias from any layer to any layer. Unless you want to pay more for blind or buried vias, the via will go through all layers but only connect to specified ones. Have a play around with the settings; it would be very hard to layout if you couldn't do that!

Changing reference planes is only necessary for impedance controlled traces (USB, HDMI, ethernet etc). For low speed things like I2C, you don't need to worry about it.

If you have very high current traces, then lay them out on the exposed layers. The main thing, as always, is to think about where current is flowing in your system. That 5 A isn't flowing everywhere; you'll have microamps into pins etc... There are plenty of calculators online for temperature rise for given trace width and position in the stackup.


Don't get too hung up about these things with your interfaces and speeds. It's more important to think about the current paths, especially as you're doing a mixed signal design.