Unfortunately, the Raspberry Pi 4 does not support upgrading RAM since the RAM chips are directly soldered onto the CPU itself as a BGA package, and are not found as separate replaceable modules. However, the Raspberry Pi 4 comes in configurations of 2GB, 4GB, and 8GB RAM so you should be able to find a configuration that suits your needs pretty easily. Just be careful about your initial choice because you can’t upgrade the memory down the line.