i doubt if there's any off the shelf solution,
like there is for 2-tones.
i'd guess you
would use 3 2-way combiners; 3 low im
circulators; several low im bpf's; a spectrum
analyzer all these components have to have
better im performance than what you're designing
i'm curious; why 4-tones? what frequency?
how low an im do you need?