FortiGate VM types

Hello sir  I am a totally novice & new at the fortigate products & im currently preparing for my NSE4(i jumped the nse1,2,3) exam. I am subscribed to the fortigate learner & i have full access for the tutorials. I like to practice while learning so i got the virtual machine fortigate. But i am getting confused at the fortigate VM types that exists. I have the 14 days trial of the fortigate for my VM but in the NSE4 study guide pdf i have noticed that the once that are being used are completely different. Mine has a red layout and it says "Fortigate VM" at the top bar while the one, being used in the tutorial has a green layout and it says "Fortigate VM-64" & it looks more advanced.  My questions are , What are the difference between the VM & VM64? Are there other versions? how can I purchase the VM64 one? Do i have to know all of the VM types & versions & how to set them up to prepare for my NSE4?   Thanks