Wikipedia tells you the answer:
The 707 uses engine-driven turbocompressors to supply pressurized air for cabin pressurization. On many commercial 707s, the outer port (number 1) engine mount is distinctly different from the other three, as this engine is not fitted with a turbocompressor. Later-model 707s typically had this configuration, although American Airlines had turbocompressors on engines 2 and 3 only. Early 707 models often had turbocompressor fairings on all four engines, but with only two or three compressors installed.
As stated above, the exact configuration varies, so some pictures show all engines the same, or not.
Further info is available from a discussion on a PPRuNe forum:
Turbocompressors were needed because insufficient bleed air was available from the engines at lower RPM's to keep the cabin pressurized during descent. Also, using bleeds reduced the climb performance greatly, whereas the TC's were more efficient at delivering cabin air. In addition, engine bleeds did not supply enough air for sufficient cabin ventilation with a full (189) pax load.
When maximum range was required, switching off all the turbocompressors and using engine bleeds for pressurization gave a 3% improvement in specific fuel consumptioin. This was OK for freighters, but resulted in a rather low cabin ventilation rate with pax aircraft.