What4 is a generic library for representing values as symbolic formulae which may contain references to symbolic values, representing unknown variables. It provides support for communicating with a variety of SAT and SMT solvers, including Z3, CVC4, CVC5, Yices, Bitwuzla, Boolector, STP, and dReal. The data representation types make heavy use of GADT-style type indices to ensure type-correct manipulation of symbolic values.