I need to transmit 100khz /90khz FSk at 9600 baud over 4km
Assuming your carrier is spectrally fairly pure (no harmonics), single-ended either RG58 or the ubiquitous RG59 will do. This site can be used for your own calculations.

Like I say, this will assume that your 90 kHz/100 kHz are pretty good sine waves. If you're using logic levels, you'll need to specify a much greater bandwidth to handle the harmonics, but even so there are coaxes available which will do the job.