Whats the difference between cd3 and cd4 cells?

There is differance: Cd3 is expressed on t-cells cd4 is a subtype of t-cell known as "helper t-cells" (cd4 cells also express cd3) cd8 is another subtype of t-cell known as "killer t-cells" (cd8 cells also express cd3).

T-cells: All t-cells in the blood have the cd3 marker on their surface. In addition to having cd3 on them, most t-cells have either CD4 (if they are helper t-cells; th2-like) or cd8 (if they are cytotoxic t-cells; th1-like). There are many other types of t-cells (nkt, th17, treg, tr1, tfoll, th9) that make up a small number of the circulating t-cells. All of these t-cells have cd3 on them as well.