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We analyze theoretically and experimentally how the modal dynamics of quantum-well semiconductor lasers is affected by weak optical feedback. Without feedback, these lasers exhibit a regular switching among several longitudinal modes, following a well determined modal sequence and leaving the total intensity output constant. Using a multimode theoretical model we have identified the four wave mixing as the dominant mechanism at the origin of these intriguing dynamics, while the asymmetry of the susceptibility function of semiconductor materials allows to explain the modal sequence. In this manuscript we show that these dynamics, which is almost insensitive to current noise or modulation, is instead extremely sensitive to optical feedback. The experimental results are satisfactorily compared with the numerical predictions of the model, properly adapted for including weak optical feedback.