Milky way kinematics. I. Measurements at the subcentral point of the fourth quadrant

We use atomic hydrogen (H I) data from the Southern Galactic Plane Survey to study the kinematics of the fourth quadrant of the Milky Way. By measuring the terminal velocity as a function of longitude throughout the fourth Galactic quadrant we have derived the most densely sampled rotation curve available for the Milky Way between 3 kpc <= R <= 8 kpc. We determine a new joint rotation curve fit for the first and fourth quadrants, which can be used for kinematic distances interior to the solar circle. From our data we place new limits on the peak-to-peak variation of streaming motions in the fourth quadrant to be similar to 10 km s(-1). We show that the shape of the average H I profile beyond the terminal velocity is consistent with gas of three velocity dispersions, a cold component with Delta(v) = 6.3 km s(-1), a warmer component with Delta(v) = 12.3 km s(-1), and a fast component with Delta(v) = 25.9 km s(-1). By examining the widths with Galactic radius we find that the narrowest two components show little variation with radius and their small-scale fluctuations track each other very well, suggesting that they share the same cloud-to-cloud motions. The width of the widest component is constant until R < 4 kpc, where it increases sharply.