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"Kilo" does not fit into the unit system of pressures in this context. As in Europe we use "bar" or "Pascal", it would either mean 4 kilobar (58015 Psi) or 4 kiloPascal (0.58 Psi).
1 bar = 100 kPa = 100000 Pa
1 Pa = 1 N/m² (apx 0.0981 kgf / m²)
1 bar = 100000 N/m² = 10 N/cm² (apx 0.981 kgf / cm²)

I would assume that Rebour is thinking in a different category in this text, that of ambient atmospheric air pressure.
Atmospheric air pressure (weather) is given a number in Europe that is around 1000. In a high pressure weather situation it is usually above 1000, in a low pressure situation it is below. The correct unit for this is millibar (mbar) or hectopascal (hPa).
So the "4 kilos" written in the text are surely 4000 mbar = 4 bar = 58 Psi.

In those days (1950!) racing bicycle tubulars on French roads were around 700x28c to 700x32c.