We numerically study the underlying nonlinear dynamics of ultra-broadband optical wavelength converters based on fourth-order scalar modulation instability. Specifically, we demonstrate the spontaneous emergence of solitons and trapped radiation waves during the frequency conversion process, leading to significant blue and red spectral expansions. Furthermore, we provide a statistical analysis of both Stokes and anti-Stokes bands, showing their intensity histogram evolution as a function of the propagation toward the L-shape signature typical of rogue events.