  We construct an infinite number of exact time dependent soliton solutions,
carrying non-trivial Hopf topological charges, in a 3+1 dimensional Lorentz
invariant theory with target space S^2. The construction is based on an ansatz
which explores the invariance of the model under the conformal group SO(4,2)
and the infinite dimensional group of area preserving diffeomorphisms of S^2.
The model is a rare example of an integrable theory in four dimensions, and the
solitons may play a role in the low energy limit of gauge theories.
