What does the "CD" of "CD4" stand for?

It stands for cluster of differentiation.

If you want to want to identify leukocytes, you can start by simply looking at them.  Often the differences are quite obvious.  For instance, to the trained eye, a neutrophil

looks nothing like a lymphocyte.

Sometimes things aren't so clear, however.  How do you tell a B lymphocyte from a T lymphocyte on microscopy?  You can't.  Furthermore, what if the cells are abnormal, or immature, as happens in haematological malignancies? 

In such cases, it's often very helpful to know the parentage of the cells of interest, and so you need to move beyond light microscopy.  One fruitful avenue to try is to look at what cell surface molecules the cells are carrying.  For instance, B lymphocytes express a different set than T lymphocytes do.  Even if a malignancy produces highly abnormal looking cells, you can often still trace its ancestry by looking for 'B cell-type' surface molecules.   This has great clinical relevance too; for instance, certain types of chemotherapy are only effective against B cell-derived malignancies.

These cell surface molecules do a wide assortment of things.  Many act as receptors, or ligands, while others have no discernible function as yet.  Whatever their day jobs, these molecules have been catalogued (for leukocytes).  Each molecule is given a number, preceded by the letters CD, or cluster of differentiation.

As an example, almost all leukocytes express CD45.  To distinguish between a B and a T lymphocyte, you need only consult the relevant table: B lymphocytes express CD19 or CD20, whereas T lymphocytes express CD3 (amongst others).

The cell surface markers are usually tested for by sending a sample (blood, bone marrow, CSF - any fluid with cells in it) for flow cytometry.