Aircraft engines are ALWAYS numbered from left to right when viewed from the pilot's seat. Additionally, the start sequence is ALMOST ALWAYS number three, four, two, one. There are several reasons for this sequence: Number three engine starter distance from the battery(s) is the shortest The longer the distance the less amps delivered. Jet aircraft like the B-707, DC-8 had the pneumatic air connection close to the Number 3 engine. Once the number three engine was started it helped pressurize the pressure manifold for assisting the additional engine starts. Remember, a low start pressure could result in a hot start. Secondly, hydraulic pumps were located on the inboards #2 & #3 engines on the DC-4, DC-6, DC-7 & DC-8 as well as the B-707 (the B-747 had hydraulic pumps on all four engines plus an APU but the start sequence remains the same). It's never a good idea to have an engine running without full hydraulic pressure available to the brakes. Thirdly, passengers or cargo is loaded from the left side of the aircraft. By starting #3 & #4 first, last minute changes could more easily be facilitated with #1 & #2 dead. Boarding an aircraft behind an idling engine is always an interesting experience. Starting the #2 next would give additional hydraulic back-up and then allow the fire guard to move to the #1 engine and exit to the left. Lastly, by delaying the start of #1 & #2 engine, interior noise would be kept at a minimum for passengers and crew until all the cabin doors were closed.