Further reading:

"...The 707s used engine-driven turbocompressors to supply high-pressure air for pressurization. The engines could not supply sufficient bleed air for this purpose without a serious loss of thrust. On many commercial 707s, the outer port (#1) engine mount is distinctly different from the other three, as this is the only engine not fitted with a turbocompressor. With engines 2 through 4 being fitted with TCs, they provide the triple redundancy required of the aircraft's cabin pressurization and air-conditioning system..." See Boeing 707 - Engines