In a cell, the endoplasmic reticulum (ER) is a vital organelle involved in many important cellular processes. One of its main functions is protein synthesis, making option 4 the correct answer.

The ER consists of two regions: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER). The RER is covered in ribosomes, which are responsible for protein synthesis. These ribosomes produce proteins that are either incorporated into the ER membrane itself or transported to other parts of the cell.

Protein synthesis is a crucial process as proteins are essential for various cellular functions. They serve as enzymes, structural components, transporters, and regulators. Without the ability to synthesize proteins, a cell would not be able to carry out its normal functions and maintain its structure.

While other cellular processes, such as carbohydrate synthesis, ATP synthesis, and fat synthesis, do occur in the cell, they are not primarily associated with the endoplasmic reticulum. Therefore, options 1, 2, and 3 are not the main functions of the ER.