D Position in 4th gear

WARNING: This page is about a different car, the 2004 Honda Odyssey, 2003 Honda Odyssey, and 2002 Honda Odyssey. However, it is still accessible from the selected car via links, so may be relevant.

Hydraulic pressure is applied to the 4th clutch, then the 4th clutch engages the mainshaft 4th gear with the mainshaft.
The mainshaft 4th gear drives the countershaft 4th gear and the countershaft.
Power is transmitted to the final drive gear, which in turn drives the final driven gear.
Hydraulic pressure is also applied to the 1st clutch, but since the rotation speed of 4th gear exceeds that of 1st gear, power from 1st gear cut off at the one-way clutch.

Fig 1: Transmission D Position Power Flow In 4th Gear
Courtesy of AMERICAN HONDA MOTOR CO., INC.