There are a couple different ways you could do it.
For example, you might make 4 inputs and send each frequency signal separately into the component. Or in the Matlab component you could implement your own filters.
In OptiSystem there is also a way to define multiple channels, differentiated by frequency, but OptiSystem will also naturally attach these channels together into one signal if the frequencies are too close. Can you let me know how far away the frequencies are from each other?
Regards