1. Red blood cells (erythrocytes): These are the most abundant type and are responsible for carrying oxygen throughout the body.

2. White blood cells (leukocytes): These are part of the immune system and fight off infections. There are several types of white blood cells, each with its own specific function.

3. Platelets (thrombocytes): These are cell fragments that help in blood clotting.

There is no fourth type of blood cell. The fourth type of blood cell is plasma, which is the liquid portion of blood that carries the blood cells, nutrients, and other substances.