ebay (via GetItDigital) has your flagship choice of the Nikon D4s or the Canon 1D X for $3,999 (the D4s retails for $6499 and 1D X retails for $5299). Both are in limited quantity while supplies last.
Check out the Nikon D4s here on ebay.
Check out the Canon 1D X here on ebay.
Note: Both are labeled as gray market items and, therefore, won’t include a US warranty card. At the time of this post, ebay seller GetItDigital has a positive feedback rating of 99.6% and offers its own 1-year warranty with the cameras.