User MadEDoctor posted on YouTube your video name, how to replace the LPDDR4 memory chip in the Raspberry Pi 4 with a larger one. However, you must be able to solder BGA (ball grid array). The price of an 8 GB chip is not that great. It can be found on Aliexpress for about 32 dollars.
It is good to test the result of the operation. It is used for this, for example memtester. In Raspberry Pi OS and elsewhere, you can easily install using sudo apt install memtester.
At the end of the video, MadEDoctor is surprised that the memtester can only test 2GB of RAM. He probably has a 32-bit version of the Raspberry Pi OS. In the 64-bit version, you can test even 8 GB at once. You just need to leave some RAM for your own OS. For 8 GB of RAM with the smallest setting of 16 MB gpu_mem it is possible to test about 7780 MB.
