Phase contrast magnetic resonance imaging (PC-MRI) is a specific type of magnetic resonance imaging used primarily to determine flow velocities. PC-MRI can be considered a method of Magnetic Resonance Velocimetry. Since modern PC-MRI is typically time-resolved, it can also be referred to as 4D imaging (three spatial dimensions plus time).

Based on that paragraph can we conclude that this sentence is true?
The difference between 3D and 4D boils down to 4D having the added element of time. 
Yes