I carburised 4140 for test bearings many years ago. We did not have any issues with carbide network formation. The carburising was at a temperature above 900 deg.C and the specimens were subsequently rehardened to give a distribution of fine spheroidal carbides. Rehardening was followed by tempering, of course.
I do not know what you mean by "liquid nitriding". Regards,Aidan Kerrigan