It's not quite accurate to say there are only "four main microbes." The microbial world is vast and diverse, encompassing a huge variety of organisms. However, we can categorize microbes into four main groups based on their cellular structure and characteristics:

1. Bacteria: These are single-celled prokaryotes, meaning they lack a nucleus and other membrane-bound organelles. They are found in virtually every environment on Earth and play vital roles in ecosystems.

2. Archaea: Also single-celled prokaryotes, archaea are distinct from bacteria in their genetic makeup and biochemistry. They often thrive in extreme environments like hot springs, salt lakes, and deep sea vents.

3. Fungi: These organisms are eukaryotes, meaning they have a nucleus and other membrane-bound organelles. Fungi can be single-celled (yeasts) or multicellular (mushrooms, molds). They are important decomposers in ecosystems and have various uses in food production and medicine.

4. Viruses: Viruses are not considered living organisms because they lack the ability to replicate on their own. They consist of a genetic material (DNA or RNA) enclosed in a protein coat. Viruses require host cells to reproduce, and they can cause a variety of diseases.

While these are the four main groups of microbes, it's important to remember that each group contains a wide variety of species, each with its own unique characteristics and roles.