You've doubtless read Hawker's sales blurb but, in case you haven't here are some snapshots:
What I can tell you for sure is that they are both fine pieces of equipment and both highly compatible in terms of performance. The Hawker may just lead the Challenger in terms of possessing a slower rate of depreciation but this, for the most part, has yet to be established as used 4000's are still somewhat scarce.