I'm planning to make 4 layer board of LPC54608.
The thickness is t1.6.
I attach the outline of athe board.
L1 is circuit layer.
L2 is GND layer.
L3 is power layer.
L4 is circuit layer.
I'm planning to empty L2,L3 under XTAL.
But I don't know how to do L4.
Should I put GND pattern in L4?
Or should I empty L4?
Please instruct me.