Oldest - 1 - 2 - 4 - 3 - youngest
Layer 1 is the oldest followed by layers 2 and 4 because of the Law of Superposition, which indicates that, when in undisturbed strata, the oldest rock layer lays at the bottom, whereas the youngest rock layer lays at the top.
The youngest layer is the layer 3 because it corresponds to an intrusion that brakes through others and appears across several other layers in a different orientation than those; the information that's given - by the layer braking through others - is that the others were already existent.
