Yes, if that suits you.
There are two ways you can do it:
Crop with nothing in the options bar, then use the Image Size function after that (with "Resample Image" checked) to resize them; or
Crop with the "px" values in the options bar.
Hey, great!
Do they mean that all the photos need to be exactly square in those dimensions?
Or do they mean that portrait photos need to be 400 high, and landscape photos need to be 400 wide? Have you taken a look at their website to check?
In fact, may I have a link to their website?
Yes, if you want to do this, that's fine. You didn't NEED to do it, because your file was already 300ppi, but if this helps you sleep better at night, go for it.
Sorry I wasn't clear. Yes, the focus is not good. You can't use this photo.
But see how her sleeve fabric texture is perfectly focused? That's what I mean by "you just missed it". Your focal plane was very very close.