After several failed attempts I managed to replace the fan of my Dell Latitude E6430 laptop. It turned out that even if the fan connector has 4 pins, the fan is actually not pulse width controlled. Instead, the fan is controlled by linear voltage regulation as a 3-wire fan. The pin-out of the connector and the colors of the wires are the following:
1 (blue): Ground
2 (red): Voltage input
3 (yellow): Sense
4 (black): Ground
Note that unlike in a normal 4-pin fan connector pin 4 is not "Control"! Instead, pin 4 is connected directly to ground (pin 1). Thus, a normal pulse width controlled fan will not work when connected to this laptop since the "control signal" is zero all the time. Nevertheless, the situation can be solved by simply cutting the control wire (usually blue). If the control signal is floating, then a pulse width controlled fan works as a linear voltage regulated one.