I don't know why they put 400W there, I think they want to be really really safe. I think it will be fine, as it is the lowest power consuming GPU out there now, and the power consumption I think should be near to 4350
Because not everyone has modern, efficient systems and because some folks stuff every possible device into their rigs. The recommended rating is just that, a recommendation, a guide, or simply something to keep in mind. Inadequate power can damage components, particularly GPUs.