I agree that a .500 spine will be closer from the jump but as you bought the .400's, do some fiddling. Download Stu's Trad Calculator (what a great tool, and a great service to the Trad world to allow free download of it rather than to sell it!).
You might be surprised what you can do by fiddling with BOP length and point weight. I've gotten Carbon Express TH 4560s(.400 area), 2213's(.500 area), 2117's, and 2314's(both .400 area) to tune to well under 1# dynamic spine/draw weight difference of virtually all of my bows by doing this.
For what it's worth, for broadheads I would rather be a touch over spined than under.
Tip: If you're using screw in points get some 5 grain brass washer weights from 3R for fine tuning. You can get 2-3 on back of most points with enough thread bite to hold the point on securely...