We discuss a theory of the coherent ultrafast nonlinear optical response of systems with strongly correlated ground states and show how the magnetoplasmon coherence in a Quantum Hall system affects the three-pulse four-wave mixing spectra. OCIS codes: (190.5970) semiconductor nonlin. optics including MQW; (320.7130) ultrafast processes in condensed matter, including semiconductor The properties of systems far from equilibrium and the role of many-body, collective, and quantum interference effects on the femtosecond and nanometer scale present relatively unexplored frontiers of condensed matter physics. A major challenge facing nanoscience is to learn how to manipulate charge and spin excitations on the nanometer scale. The cold 2DEG in modulation doped quantum wells (MDQW) in a magnetic field provides a clean system for testing out new ideas.A large magnetic field along the growth axis of a MDQW creates discrete Landau levels (LLs) partially filled with electrons introduced by doping. It thus appears that it should be possible to describe the nonlinear optical properties of this system with few level system models similar to atomic physics. It turns out however that the highly degenerate LL states are coupled by electron-electron interaction, which breaks the degeneracy and leads to novel Quantum Hall many-body effects (QHE) and collective electronic excitations such as the inter-LL magnetoplasmons (MP) [1].One of the challenges facing the study of the ultrafast nonlinear optical response is to develop a theory that applies to systems with a strongly correlated ground state, such as the Quantum Hall system. In such a system, standard diagrammatic expansions that assume a Hartree-Fock ground state break down and the approximations upon which the Dynamics Controlled Truncation Scheme (DCTS) [2] is based must be reexamined. In a recent work [3-6] we introduced such a theory, which recovers the DCTS results in the undoped system while including the ground state correlations. Here we use this theory to study the role of the magnetoplasmon coherence in three-pulse FWM spectroscopy. In this case, two optical pulses excite the 2DEG via a second order Raman process assisted by Coulomb interactions and can create coherence between the lowest (LLO) and next lowest (LL1) Landau levels. Subsequently, the third pulse scatters off this coherence, leading to FWM signal. The above process interferes with the usual Pauli blocking (PSF) effects which leads to beatings described below. In addition, the LL1 magnetoexciton (MX) can scatter with the 2DEG and decay into a LLO MX and a magnetoplasmon. This leads to a strong nonMarkovian LLI dephasing and an asymmetric lineshape. In contrast to LL1, an analogous scattering process for LLO is nonresonant and thus the LLO dephasing time is much longer.Our theory reduces to the following set of equations close to the Quantum Hall Ferromagnet regime, where the ground state consists of spin up electrons while photoexcitation with right circularly polarized light excites spindown electrons:ia P...