  N=2 extension of affine algebra $\hat{sl(2)\oplus u(1)}$ possesses a hidden
global N=4 supersymmetry and provides a second hamiltonian structure for a new
N=4 supersymmetric integrable hierarchy defined on N=2 affine supercurrents.
This system is an N=4 extension of at once two hierarchies, N=2 NLS and N=2
mKdV ones. It is related to N=4 KdV hierarchy via a generalized
Sugawara-Feigin-Fuks construction which relates N=2 $\hat{sl(2)\oplus u(1)}$
algebra to ``small'' N=4 SCA. We also find the underlying affine hierarchy for
another integrable system with the N=4 SCA second hamiltonian structure,
``quasi'' N=4 KdV hierarchy. It respects only N=2 supersymmetry. For both new
hierarchies we construct scalar Lax formulations. We speculate that any N=2
affine algebra admitting a quaternionic structure possesses N=4 supersymmetry
and so can be used to produce N=4 supersymmetric hierarchies. This suggests a
way of classifying all such hierarchies.
