4.5 gal to 3.7. That sucks. The one advantage the 300 has had over the other small engine makes that I've liked is a larger tank and longer range. Although my CB500X is giving me a few miles more out of a tank than my old 300 and they both hold 4.5 gal.
Looks like the airbox on the 400 is underneath the gas tank taking away volume for gas. It also looks like you would have to remove the gas tank to clean the air filter.