make sure each injector has a 12v supply with the ignition on,disconnect the injector plug and probe one of the pins in the plug for 12v,if nothing then check for a bad connection at the square connector block on the bulkhead,the heavy red wire into the block is the injector supply,
although unusual for 4 injectors not to opperate,its possible that the ecu is not telling them to open ie giving them an earth in sequence,this as far as i,m aware is controled by the phase sensor in the dizzy,so a good check the phase sensor gap and of all the connections and plugs in this area is recomended,
incidently how do you know the injectors are not opening?are the spark plugs dry,is there no smell of raw fuel from the exhaust,are u sure its not spark related