Originally, Channel Television picked up their feed of the ITV Network from Westward Television (based in Plymouth), but they had an agreement with Southern Television to take their some of their programmes too. Remember that regional schedules were very different from each other in those days! They received the incoming feeds off-air with television aerials pointing at the nearest mainland transmitters.
However, in 1982, both Westward and Southern lost their licenses (to TSW and TVS respectively). Channel TV set up an arrangement to take an off-air feed from TSW, but the new Plymouth broadcaster turned their schedule upside down, replacing popular networked shows with regional programmes and moving programmes to odd slots. Crossroads, for instance, was moved from 6.35pm to 5.20pm causing a good deal of protest on the islands. Channel arranged to tape such programmes and put them out in the original slot until they reached a new agreement with TVS in 1985 to take the network feed from them. TVS lost their franchise in 1992 and Channel's Master feed then came from TVS's successor Meridian.
Much of this information is taken from
The Continuity Booth
Now returning to the threads actualy subject, presumably Channel's early Channel Four relays came from TSW until 1985 and then from TVS until Channel Four took over the transmission of it's own advertisements, when I suppose it will have become part of C4's Southern region.