Cell theory is one of the fundamental concepts in biology, which describes the basic unit of life. It states that all living organisms are composed of cells and that cells are the fundamental unit of life. The theory was developed over time by different scientists who made various contributions to the understanding of cells.

There are three principles of cell theory, which are universally accepted by scientists. These principles are as follows:

1. All living organisms are composed of one or more cells: This principle states that all living things, including plants, animals, and microorganisms, are made up of cells.

2. Cells are the basic unit of life: This principle states that cells are the smallest unit of life and that all the functions of living organisms occur within cells.

3. All cells arise from pre-existing cells: This principle states that new cells can only arise from existing cells through cell division.

However, there is no fourth principle to cell theory; hence none other than these three principles can be considered a part of cell theory.

It is important to note that there have been many advancements and discoveries since the development of cell theory. For instance, scientists have discovered different types of cells such as prokaryotic and eukaryotic cells, which differ in their structure and function. Additionally, science has also shown us how some viruses can replicate without using a host cell; hence they do not fit into the definition of a living organism.

In summary, there is no fourth principle to cell theory; hence it remains as a fundamental concept in biology with its three universally accepted principles. However, advancements in science continue to reveal new insights about cells and their functions, shaping our understanding of life itself.