Do not use hex formulations, e.g., ELFORM 1,2, or 3, for tetrahedrons.
Tet form 10 is available for implicit analysis (960/rev.1488). In addition, tet form 4 and the 10-noded tet form 16 are available for implicit in version 970.
An implicit solution using tets will always be too stiff though accuracy will generally improve as you increase the number of elements through the thickness. Using the elastic cantilever model, loading in the strong (y) direction gives a reasonable solution (10% too stiff for tet 10; 2% for tet 4) while loading in the weak (z) direction gives a solution which is much too stiff. Even for the y-direction loading condition, the stress distribution in the tet mesh is somewhat irregular as compared to the stress distribution in a hex mesh.
In summary, use of tets is generally discouraged, particularly in implicit simulations. The one case I can think of where tets may be beneficial in modeling of highly deformable, soft foam which may otherwise become unstable if modeled using hex elements.