In 4th Gear

Hydraulic pressure is applied to the 4th clutch, then the 4th clutch engages the secondary shaft 4th gear with the secondary shaft.
The mainshaft idler gear drives the secondary shaft via the idler gear shaft idler gear and the secondary shaft idler gear.
The secondary shaft 4th gear drives the countershaft 4th-5th gear.
The countershaft 4th-5th gear drives the countershaft via the reverse selector, which drives the reverse selector hub.
Power is transmitted to the final drive gear, which in turn drives the final driven gear.

Fig 1: Power Flow - In 4Th Gear
Courtesy of AMERICAN HONDA MOTOR CO., INC.