In the forging process, several steps occur that help shape the final product. Let's go through each option and explain why they are correct.

1. The metal structure is refined:
In forging, the metal structure undergoes refinement. This happens due to the application of heat and pressure during the process. As the metal gets heated, the grain boundaries recrystallize, resulting in a finer grain structure. This refinement leads to improved mechanical properties of the material, such as increased strength and toughness.

2. Original unidirectional fibers are distorted:
During forging, the original unidirectional fibers present in the starting material are distorted. This happens because of the plastic deformation of the metal under the applied pressure. The transformation of the original fiber structure occurs as the material is shaped into the desired form. The distortion of these fibers helps realign the material's internal structure, reducing anisotropy and improving the overall mechanical properties.

3. Poor reliability, as flaws are always there due to intense working:
When metal is subjected to intense working in the forging process, it can result in the introduction of flaws and defects. These flaws can include cracks, voids, or inclusions, which may affect the reliability of the final product. The intense working can cause localized stress concentrations, leading to the initiation and propagation of defects. Therefore, poor reliability is a possible consequence of the forging process.

4. Part is shaped by plastic deformation of the material:
In forging, the part is shaped by the plastic deformation of the material. Plastic deformation refers to the permanent change in shape that occurs when a material is subjected to external forces beyond its elastic limit. The application of pressure and heat during forging causes the material to undergo plastic deformation, allowing it to be shaped into the desired form. This plastic deformation is responsible for the change in shape and the improvement in mechanical properties.

Therefore, based on the explanations provided above, the correct answer is option 'C' - 1, 2, and 4.