First of all, we must understand the meaning of 2G, 3G, 4G, G refers to generation, their meaning is the second, third, fourth generation of mobile communication technology\/standards.
3G mobile phone refers to the highest support for the mainstream 3G standard mobile phone, that is, WCDMA, CDMA2000 or TD-SCDMA, then obviously it is not able to support some 4G technology, even if you use 4G card, it is only because it is compatible with the previous generations of communication technology. As for whether the 3G Unicom SIM card can access the 4G network, I think it depends on whether the operator gets through the relevant links, and the user identity module and encryption and decryption algorithm of the SIM card may not be the key. The above is purely theoretical.