  In a previous paper we have proposed a new method for proving the existence
of "canard solutions" for three and four-dimensional singularly perturbed
systems with only one fast variable which improves the methods used until now.
The aim of this work is to extend this method to the case of four-dimensional
singularly perturbed systems with two slow and two fast variables. This method
enables to state a unique generic condition for the existence of "canard
solutions" for such four-dimensional singularly perturbed systems which is
based on the stability of folded singularities (pseudo singular points in this
case) of the normalized slow dynamics deduced from a well-known property of
linear algebra. This unique generic condition is identical to that provided in
previous works. Applications of this method to the famous coupled
FitzHugh-Nagumo equations and to the Hodgkin-Huxley model enables to show the
existence of "canard solutions" in such systems.

Chaotic Dynamics;Dynamical Systems