The T-4 program can be thought of as something of a precursor to the Holocaust proper. It was an early Nazi program of "euthanasia" that was targeted at people who were seen as in some way substandard by the Nazi regime.
In general, the people who were killed in this program were those who were not likely to be of economic value to the Reich or who were seen as somehow "weakening" the race. The victims of the T-4 program included people who were patients in mental asylums and people who simply had physical diseases such as epilepsy.
In some ways, this can be seen as a rehearsal for the Holocaust. Many of the same techniques used in the T-4 program were used in the Holocaust as well. The main example of this is the fact that the victims were often killed by gas in facilities that were supposedly showers soon after they arrived at the killing centers.
The T-4 program, then, was a precursor to the Holocaust in which the victims were mainly people who were seen as worthless to the Nazi regime.