Question

Which stars in the Hertzsprung-Russell diagram have surface temperatures that are all above 4,000°C?

Answer 1

See below:

Explanation:

Here’s a common version of the H-R diagram:

As you can see, the short answer is ‘many of them’, but the questioner is probably looking for the answer “white dwarves” (and possibly “the variables”.) All other categories have such a large range of temperatures that they don’t exclusively have `T >= 4000`K and since you specifically asked for stars who are all above that temperature the answer is more limited.