  We compare phase matching for four-wave mixing using one, two, three, and
four waveguide modes. For the comparison, we use numerical optimizations and an
estimate of the generated idler power. We present results for few-mode fibers
and nano-rib waveguides and show that for both waveguide types, four-wave
mixing bandwidths and idler powers are best for one- and two-mode operation and
that four-mode four-wave mixing is not feasible at all. Some nano-rib
waveguides support three-mode four-wave mixing, albeit with much reduced
bandwidth and reduced idler power.
