After the sales of RD350 went down in mid 70th, the company decided to punch the engine out to 400cc and make some other improvements in order to produce an easily manageable, but at the same time more powerful and well responsive motorcycle. It was definitely a good decision at the time.
Yamaha RD400 came with an increased stroke, a more attractive sporty look and some impressive innovations, like cast discs and new brakes. By the way, RD400 was the first productive motorcycle wich had been initially designed with cast discs, as a standard fitment.
The rubber-mounted engine technique greatly reduced the vibration. Moreover, the engine was relocated a little bit forward to move the center of gravity from the back to the front. The engine design required certain modifications to fit an extra 50 (48) cubic centimeters. Ironically, that's what would cause some serious problems with the engine wear in the future.
Yamaha RD400 was famous for its acceleration that, in fact, wasn't that beneficial in traffic, as the bike responded pretty aggressively and could have been hard to control. Another problem was the brakes that didn't always do the job well and often couldn't stop the bike as quickly as it was needed.
Despite all the performance problems that had been faced, Yamaha RD400 had a good run and, what's even more important, opened the door for a new better RD350LC. The production of RD400 was discontinued in 1979.