I have just purchased an XVS1300A and was informed by the Yamaha Dealer that 4th and 5th gears are OVERDRIVES, you'll find this info in the handbook also.
When riding I find the bike chugs at low rev's for example up to 60kph (35mph) I only use 1st or 2nd gear, then when above this speed I change up to 3rd and dont engage 4th gear until above 80kph (50mph) I use 5th gear at 100kph (60mph).
You can use say 5th gear at 80kph (50mph) but when you go to accelerate it WILL CHUG hence ChANGE DOWN gear.