After you press the 4 key then press and hold the Right mouse button. That will move you around. In settings you can change the speed of movement. It's take some getting used to. Your starting point is the center of the screen. NOT where you initially click. So try to initially click and hold in the center of the screen then move mouse. You can also use arrow keys while moving to spin left or right or change the "plane" of your view. Like I said, it takes getting used to.
You can just rotate around your train by leaving focus on loco (hit 2 key) then right click, hold, and spin around with mouse movement. Use mouse scroll to zoom in and out.