It's not a big deal if you submit 5k instead of 4k; we're pretty flexible. The 4k limit is just a guideline to keep us from getting inundated with novella-sized submissions every week.
I think trying to cap submissions by word count would be difficult (especially since a lot of people are writing their submissions the weekend or even the Monday morning before they submit), and it might end up limiting the number of people who could submit in a week unduly. Maybe for weeks when we have fewer submitters (say, three or less) we could up the word cap for their submissions or something so they can push more material through. If people are willing to give up their quieter weeks, of course.
The crazy busy weeks happen, but not very often, and mostly I think it just means that people are a little slower getting to critiques, not that they don't get to them at all.
4-5k isn't a lot, it's true, but there are a lot of us. Having submissions of 8-10k would add up in a real hurry with just five people submitting a week; at that point you're practically critiquing a book a week, and I suspect a lot of people wouldn't be able to keep up. And I think most people don't want to see the number of crits they get a week go down.