A Steam Deck user managed to run the system at 4k and 60fps using an external graphics card. It's quite an achievement, but not recommended.

ETA Prime, a YouTuber, has produced a 10-minute video describing the procedures he took to get everything running. By unlocking the console and removing the SSD storage from its M.2 expansion slot, ETA Prime was able to install a Radeon RX 6900 XT GPU.

He taped the GPU to an adaptor and connect it to an external power source to do so.  A portable system with a GPU coupled to the Steam Deck is no longer portable, according to ETA Prime. Why will be the first question. "'Because we can'." After removing the SSD card, ETA Prime had to boot the console using a Micro SD card with Windows 11 and its games on an external USB-C SSD.

On Ultra settings, he was able to get The Witcher III to run at 4K resolution and 108fps after attaching the Steam Deck to a display and starting it. He also got GTA V to run at roughly 70 fps on ultra 4K settings.

He also says that the results are ultimately limited by the Steam Deck's internal CPU, and that newer games will be limited regardless of the linked GPU's capabilities. To show, he ran Elden Ring at 1080p resolution at full settings and got roughly 46fps, but Cyberpunk 2077 gets 40 to 80fps. Finally, ETA Prime feels no one else should try the experiment due to its complexity. "I wouldn't endorse it," he says. "The Steam Deck is transportable." This was just a test to see if it worked, and it did."

User mod: Steam Deck achieves 4K resolution and 60 FPS with Radeon RX 6900 XT externally