Hello XMBOND and welcome to these forums. You'll find many helpful people here and a lot of useful information. Make sure that you:
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Iin Reaper's Preferences, configure your Audio Output device to use all 4 channels.
On the master track, click the [IO] button, set the number of channels to 4 and add a new HW output, from channels 3+4 to the Behringer's 3+4 channels:
I do not know if you have to do anything else in OSX