This paper studies the non-linear tolerance of several modulation formats in a four-span WDM system (8x10Gb/s) using low chromatic dispersion fiber. The narrow spacing between the channels (50 GHz) makes FWM to be the most detrimental effect experienced in each span made of G.655 fiber (80 km, D=2ps/nm.km) compensated by 0.8 km of DCF. A particular attention to the Q factor computed in the simulations enables a fair comparison between IMDD (Intensity Modulated Direct Detection) and phase modulated formats. It is shown that the various amplitude modulation alternatives result in more or less the same performance. Phase modulation schemes drastically increase the system performance leading to an increase of the Q -factor by almost 3dB.