Was the sword forging scene at the beginning of season 4 CGI or real?

Most likely, the scene is a mix of real shots and special effects, but all of the cool parts are special effects.

For reference, here is an image of the scene in question (I was unable to upload it to SE due to the file size being too large).

Heating of the sword

The glowing effect is CGI. I say this because the sword has a consistent yellow and orange glow to it.

During this kind of metal-smithing, the metal is usually somewhat unevenly heated. Because of this, you end up with a red color on the edges, and if heated enough, parts of the metal turn white as well.

Also, notice how fast the glow happens. As we pan down on the sword, it slowly turns orange, but then all of a sudden the brightness increases dramatically. This just wouldn't happen in real life.

The fire on the other hand does look real. Fire, as a particle system is notoriously difficult to render realistically. It would be pretty cheap and simple just to light a couple small fires underneath the surface that the sword is laying on.

Pouring the steel

A Foundry Ladle is used as a container to melt the steel, then pour it into the mold. Typically, they are made of metal, ceramic, or concrete materials. In other words, heavy materials that are resistant to heat. But, you can clearly see ladle in this scene is glowing through the sides. To get that kind of glow, the ladle would have to be a light semi-opaque material. Then, the glowing would be accomplished by placing lights inside.

It was real but it wasn't the right way to make a sword, and it wasn't steel, when steel it's got enough to turn into a liquid, it is a bright white, that is about 1200°f, the stone would cool the steel causing it to create a loo that stopped it from flowing to the top, and it would spill out of the top of the cast, if it was steel they would have hammered it into shape, as the cast would have made one side of the sword completely flat.

What they used was molten aluminum, no CG required, and it is a lot cheaper as they probably had aluminum cans saved up from the refreshments table, the finished sword they use were properly forged off camera with a hammer using the modern pattern welding technique. That is when you take a lot of thin diffrent grades of steel, and forge them together, then dip them in acid to give that Damascus wavy pattern look.