Two things specific to the RX 6400 worth knowing:

• It's a PCIe 4.0 x4 card. On a PCIe 3.0 board the link is half as fast, and that hurts most exactly when a game spills past the 4 GB and starts moving textures over the bus. So on a 3.0 board, keeping texture quality low enough to stay inside VRAM matters even more than usual. MangoHud can show VRAM use live, so you can see when you hit the ceiling.
• For emulation, VRAM mostly goes on internal resolution and texture packs. In Dolphin, PCSX2 and the like, 2x-3x internal res is usually comfortable on 4 GB; 4x+ with high-res texture packs is where you'd start running out.

It also has no hardware video encoder, so don't count on it for recording or streaming, but for older games and emulation it's a perfectly decent card.