Pipes may fit - check the o/d & i/d of the pipes where they go on to the cruciform section of the downpipes - if they match then the rest of the bike is the same so you should be ok.
ECU definitely no - ST3 uses the U59 ecu which is flashed at the factory with the fuekl / injection maps and not able to be modified, totally differnet to the ST4 which uses the 16M with a removeable eprom.