It's more powerful, has upgraded sights, is really compact, has the mag in the stock like the first gun, removable mags, etc.
Unfortunately it doesn't have a ram guide (in development), and is less accurate.
The first pics are the R4, the last is the R3.
You can find a short video of it shooting on my photobucket. I will make a better video later.
EDIT: IT IS NOW POSTED!!!!