Running 2K resolution on 4K monitor?

ok guys… here is something i ran into by accident. let me explain… Or skip to the last sentence… I used to run 3 40" 1080p tvs spanned into 1 with NVidia 1080 on my old pc. Bought a new one for fs20. 9900k 4.68g 32g 2080ti with m2 drives. I found spanning 3 display’s in fs20 was frame hitter… meaning it gets choppy landing at LAX with high settings. so i went to 1 1080p display with trackir. Walmart has a 50" 4k tv on sale for $200. made me wonder how this sim will run in 4k. to test it i ran a long hdmi cable to the mancave big screen tv. 65" 4k. I ran great… minimal frame hit with beautiful graphics. obviously im not gonna run that big display 18" in front of my face. so i shut everything down and hooked my 1080p back up. fired up the pc and made sure all my display were correct then fired up fs20. I was in developer mode to test the higher resolution when i quit the sim to reboot. when the sim came back up on my 1080p display i could hardly read the developer menu bar. so i checked the sim settings and it remembers my last 2160p setting even tho w10 says its a 1080p display. so now i have 4k resolution on a 1080p display. ini i can do that in sim by sliding over the resolution but thats a bigtime frame hitter. im running 4k with zero frame hit at 100 slider setting on a 1080p display.