The US and EU stock antennas look shorter than the CN antenna. Are all of them 4dBi?

Yes. All of our stock antennas are 4dBi. The length of the antenna is proportional to the wavelength of the frequency band. The frequency wavelength of 470 mHz is roughly twice that of 915 mHz, so proportionally, the CN 470 stock antenna is longer than the US/EU stock antenna.