It appears that GPD may not be equipping the Win 4 with native landscape displays. Instead, the company is using IC conversion which is causing micro-stuttering when playing 2D and 3D games. Apparently, GPD is actively working to resolve the issue.https://www.notebookcheck.net/GPD-Win-4-Source-of-in-game-micro-stutters-surfaces-for-new-gaming-handheld.709867.0.html
GPD's reply is one of the worst I've seen. They are definitely losing customers for this.
Phawx's comment about lowering the refresh rate "fixes it completely" is completely incorrect.
GPD released an official fix for this and a few of us on the GPD discord have successfully flashed the IC chip. No need to solder but you do have to open the back of the device.
Yesterday, GPD released a software only fix. Yes, it updates the screens FW. I've used it on mine and it completely solved the issue.
Droix published an excellent guide to follow.