it's not possible to place power only traces on 3rd layer and signals on 4th because there will be some slots here and there and it will make my current loop alot longer.
1 : Components, short traces (10 mm or less, say)
2 : Power
3 : Ground
4 : Any length traces
By using 1 and 4 as trace layers; and 2 and 3 as plane layers you make a ...
Question is should I use 3rd layer as a power/signal and 4th as a big
ground pour? Or should I swap it and make 3rd layer as a GND and 4th
as a signal/power.
It is usually better to put the a solid GND on one of the inner layers.
1) it facilitates a nice ground plane and creates a small amount of capacitance between the traces on the outer layer, ...
I would go with this :
Signal - Power
Power distribution & ground
Signal - power
Signal on top is obvious, it is to avoid vias as much as possible.
Ground on layer 2 is obvious too.
I usually use Layer 3 for power distribution, it helps a ton to keep the signal layer as clean as possible, and allows optimal routing of power traces.
I try ...