Having ridden both. The Ninja300 felt way lighter than a CB400. I urge you to try it yourself. Also try the acceleration and throttle response in real world situations. Like stop/go or low to medium speed overtake. The parallel twin really shines over the I4.
If you still feel the CB400 is a better and quicker bike, then feel free to pay more for it (considering it's age and cost)

BTW you quoted the wet weight for the EX300 but dry weight for CB400.