It's a common part of the writing process, I think. Happens to me all the time -- and often when I look at the stuff again, later, I realise that actually there was some merit in the original material, it just needed some minor tweaking.

I'd suggest starting a new page and writing a new scene to get your 400 words, without looking back at the old work. Then putting it away for at least a few days to get a bit more perspective on it.