Well if Crake can monitor the signal OK I’m guessing the deck has a preamp built in or they are using a preamp of some kind.
Inputs 3/4 are available to the PC but probably it needs to be enabled in whatever software you are recording into (streaming from in this case).
I would check the forums of the broadcast software for how to mix and stream multiple inputs.
If they do not have these options you could certainly do it with something like this: