All four 1 kW amps will need to be driven by the same r-f source, with the same r-f phase. Normally that is done by splitting the output power of a single FM exciter, by one of various methods. So the exciter must be rated to supply the total output power needed at the inputs of all four 1 kW amplifiers, plus splitter and cable loss.
Then probably the cleanest way to combine the amplifiers while supplying the isolation needed between their output circuits is with three 3 dB hybrids and associated reject (dump) loads. There are other issues in doing this such as providing common control, r-f monitoring and protection circuits for the system.
This system design and test, and the FCC documentation needed to use it legally on the air will not be a trivial exercise.
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