Thanks Colin.
The Matrix 32 add-in works really well for the purpose. My internet research didn't turn up as much with "quartic equation solving", but I did find (http://www.mrexcel.com/forum/showthread.php?t=88804) which is nice quick code, but it can't handle complex numbers. Hopefully, Matrix32 covers all !
(SOLVER works too, but only finds the first root!)