  Relativistic massless Weyl and Dirac fermions have isotropic and linear
dispersion relations to maintain Poincar\'{e} symmetry, which is the most basic
symmetry in high-energy physics. The situation in condensed matter physics is
less constrained; only certain subgroups of Poincar\'{e} symmetry -- the 230
space groups that exist in 3D lattices -- need be respected. Then, the free
fermionic excitations that have no high energy analogues could exist in solid
state systems. Here, We discovered a type of nonlinear Dirac fermion without
high-energy analogue in SrAgBi and named it type-IV Dirac fermion. The type-IV
Dirac fermion has a nonlinear dispersion relationship and is similar to the
type-II Dirac fermion, which has electron pocket and hole pocket. The effective
model for the type-IV Dirac fermion is also found. It is worth pointing out
that there is a type-II Dirac fermion near this new Dirac fermion. So we used
two models to describe the coexistence of these two Dirac fermions. Topological
surface states of these two Dirac points are also calculated. We envision that
our findings will stimulate researchers to study novel physics of type-IV Dirac
fermions, as well as the interplay of type-II and type-IV Dirac fermions.
