Simply put, 4K Ultra HD is just a higher resolution; roughly four times higher than regular HD. 4K Ultra HD has 3840 x 2160 resolution and creates images that are crisper and capable of showing more details than HD (which is 1920 x 1080 resolution.) 4K Ultra projectors are still very new to the marketplace, so they’re hard to find, and when you do, they’re quite expensive.
How does LED fit in? LED’s biggest advantage is its effectiveness with ambient light. If you’re working in a space with a lot of natural light, like ballroom foyers, LED will likely be a better solution than projection. If your event is outside during the day, an LED screen is essentially your only choice because projection is almost never powerful enough to compete with direct sunlight.