Unfortunately they are just two different marketing wordings for the same thing. The 750M is a "dedicated" GPU, and it just means it doesn't share RAM with the CPU. It is also an "integrated" graphics card for laptops.
Go for the 4gb version. If they're roughly the same price budget, might as well get more for less. If it's significantly more expensive, go for the 2GB Version, as the 750M is most likely going to bottleneck performance far before you hit your video RAM limit.