What is the simplest way or best topology for an 9v dc battery input to generate four 2.5v dc/250mA each isolated outputs?
Fastest way is: Get four DC-DC isolated 1W converters like this one from Recom, then add a SWITCHING DC-DC regulator from 5 V to 2.5V (fully integrated, >90% efficient). You should manage to get 300+ mA from each output.