The fact that it's a 4" pipe that the sump goes into would suggest that
it's possible it was tied into the sanitary sewer system, perhaps
through a clean out fitting. That is not allowed in most areas.
However the fact that water won't go in it argues against that. What's
on the other side of the basement wall? Does the sewer exit through
that wall in that location? If not, then it's probably not tied into
the sewer. Could be just a whacko hook up to an underground drywell
that failed, though it's unusual to use 4" pipe for that.

But probably doesn't matter. If you have a good discharge spot 20 ft
away, I'd just permanently use that.