Ceilidh, for someone who said he flew 707, it's a quite poor explanation.
In fact due to FAA certification in the 50's they do not allow air conditionning and presurization air to be picked directly from the compressor of the engine. They fear to have a mixed of engine oil in this air. So the imposed to have an high pressure bleed air turbocompressor wich take air directly outside the engine.
Here is exactly what is written in the PAN AM 707 Operation Manual (Pneumatic System 707.161 p1 -- Sep13, 1974) :
After the engine are started, the primary pneumatic source is from engine air-driven turbocompressors. Ambient air enters a turbocompressor ram inlet on top of the engine nacelle, is compressed, and is then routed to the manifold.
Why is there a difference between the engines mount?
Simply because there is only THREE turbocompressors, mounted only on engines 2,3 and 4.
Hope it helps.Rgds.
Sabena ... Never to be forgotten (12 years already , what a shame !! )