Can anyone teach me how to make a fog that blocks/limits the sight of the player?

I wanted to make a fog like from silent hills. i want it to slowly load the area as you go through it.
link text

You can use Exponential Height Fog to do this. Just drag it from the Modes / Volumes panel into your level. Even though it’s called “height fog”, you can use it as general distance-based fog by just placing it high above the camera so that the camera is deep inside the fog. Set Fog Max Opacity to 1, Start Distance to 0, and crank up the Fog Density (you can type larger values than the slider lets you).

Will this make the loading of the area being seen go faster?

I don’t believe so.

i was hoping for something that might be able to help the game run on low end pc as well. can you give some help on that one?

If you want to limit the number of objects drawn, you can use a Cull Distance Volume in conjunction with your dense fog volume. Cull Distance Volumes remove objects smaller than a provided size that are farther than a provided distance. For your application, you can tell the Cull Distance Volume to cull all objects that are further than whatever distance your fog allows you to see.

UE4 has many settings that you can adjust to throttle performance for low-end PCs. Here’s a great reference: