Does RAM capacity limit also includes the type?

[DavidVioMC]

I got a Dell Optiplex 960 SFF and in the manual it says that you can only put max 8GB of RAM, I currently have the full 8GB DDR2 ram but I want to put DDR3/4, will that be fine?

 

[SkyNetRising]

Of course not.
DDR, DDR2, DDR3, DDR4 each have different slots. Google for differences between them.