There are two aspects to this issue :
How do you get the four tuners to work independently. You can do this with a Quad or Quattro LNB (with 4 cables to the R1), or a Unicable/Unicable II LNB or Multiswitch (with one cable) - though that latter may require you to use third party firmware like SatAxe and to understand how to configure Unicable/Unicable II. This will allow you to tune to a maximum of 4 satellite transponders.
How many channels that you want to provide to your network does each transponder have? You can use one tuner and LNB feed/Unicable UB slot to tune more than one channel if it's on the same transponder (though you may not be able to stream ALL of the channels on a transponder). When you tune to a transponder, the R1 will get a data stream with all the channels in that transponder, and will filter out the ones you want. You can usually filter out a number of channels from the same transponder before you hit a processing limit (it depends how hard the R1 is working I think)
If you want to offer specific channels to your network - you can look at which channels are on which transponder and come up with a plan to use your 4 tuners to decide which 4 transponders make the most sense. Or you can just decide to limit each user to one tuner/transponder and limit it to 4 users. They are two sensible approaches for different reasons.