According to the time traveler, what is the fourth dimension?

The TT explains that Time adds a fourth dimension to the three dimensions of space. We overlook the fourth dimension because "our consciousness moves along it." He argues that Time, therefore, is a dimension of space. The Medical Man believes the difference between Space and Time is that one cannot move in Time and "get away from the present moment." The TT counters that we are always doing just that, "passing along the Time-Dimension with a uniform velocity." The fourth dimension is Time.