The Mendelssohn, Respighi, Dvorak, Debussy, Ravel, and Smetana are all arranged/originally composed for four hands by the composers themselves, yes.
The Mozart Symphonies are arranged by someone else I believe, but the arrangements are very good. If you want original, Mozart wrote many fantastic Sonatas for Four Hands, as you well know.
The Mendelssohn, Respighi, Dvorak, Debussy, Ravel, and Smetana are all among the finest musical compositions in the entire repertoire. Beware though.... four hands playing at this level requires the utmost musical virtuosity from both the primo and secondo players!
You need a partner who is at the same level as you, and both you and your partner will have to be amazingly good to be able to pull off this kind of repertoire.
I also highly recommend the Grand Duo Sonata by Schubert. It is one of the largest and grandest original four hands works in the entire repertoire. Much, much harder and more beautiful than the F minor Fantasy.