The specification exactly means that by varying the tuning voltage by one volt, the VCO frequency varies by 400 MHz. One millivolt change will result in 400 kHz frequency change.
Use a potentiometer as voltage divider to get the sweep you need. As the voltage variations are quite small, use regulated voltage and a good screening of tuning circuit.
The reverse situation would call for amplification of tuning voltage. You are on a lucky side!