Go into the BIOS/UEFI. Look for "UMA Frame buffer Size". Reduce the buffer size. The BIOS is reserving 2GB of main memory for the 3400g.
Ryzen 5 Pro 4650g, ASRock B550 Phantom Gaming 4/ac, 16GB Crucial Ballistix 3600MHz, SAMSUNG 970 EVO NVMe 500GB, Super Flower Leadex III Gold 850W