We report experimentally on the phenomenon of intra-envelope four-wave mixing (FWM) in optical fibers. We demonstrate that the non-linear interaction between two pulse trains operating at the same repetition rate, but with slightly different carrier frequencies, leads by FWM to the generation of new components within their spectral envelops. The frequency differences between the FWM components and the pump lines are in the RF domain, making it almost impossible to observe the process using a standard optical spectrum analyzer. We show that using a third light source as a multi-line local oscillator makes the analysis of these new spectral components accessible. It enables to extract these FWM components from the spectra of the initial light sources by converting optical frequencies into radio frequencies. The experimental results are in excellent agreement with the numerical simulations.