Luckily, you don't need to decide whether to go with OLED or 4K – you can have both! 4K refers to the resolution (ie, the number of pixels on the screen), while OLED refers to the panel technology (ie, what the pixels are made from).
So you can have 4K screens that are OLED (almost all of them are, apart from some pricey 8K OLED TVs), and you can have 4K screens that are QLED, mini-LED or other tech.