The 8th digit of the Vehicle Identification Number indicates what engine the car has.

The Difference Between the AX4S and AX4N

D= 4 cylinder (2.5L OHV)
4= Essex (3.8 OHV)
S= Duratec (3.0 DOHC)
U= Vulcan (3.0 OHV)
1= FFV Vulcan (3.0 OHV)1
2= FFV Vulcan (3.0 OHV)2
Y= V6 SHO Manual (3.0 DOHC)
P= V6 SHO Auto (3.2 DOHC)
N= V8 SHO (3.4 DOHC)
W= Cyclone (3.5L DOHC)
T= EcoBoost (3.5L DOHC)

1 Methanol     2 E85/Ethanol