Yes 7990 does perform better in 4K acording to benchmarks by this Korean website. Nevermind if you don't understand the articles. The benchmark graphs say it all.
http://udteam.tistory.com/565
Here another comparison with GTX 690 by Engadget http://www.engadget.com/2013/04/24/amd-details-radeon-hd-7990/
But 7990 consumes a bit more power tho. http://www.guru3d.com/articles_pages/radeon_hd_7990_review,8.html