If anything vs If something

Which is correct?

1. Please let us know if we need to do anything about it.
2. Please let us know if we need to do something about it.

Which is correct?

1. Please let us know if we need to do anything about it.
2. Please let us know if we need to do something about it.

Both sentences are correct and can be interpreted only existentially, with the first more natural. Although the second sentence is slightly marked, in the wild I'd treat them as functionally identical rather than risk overreading.

By contrast, the sentences

3. Please let us know if you can eat anything.
4. Please let us know if you can eat something.

are more genuinely divergent in possible interpretation. While Sentence 4 can be interpreted only existentially, Sentence 3, in the absence of prosody and context, is arguably ambiguous between the existential reading

3a. Please let us know if you can eat anything (even if it is just a few crackers).

and the universal reading

3b. Please let us know if you can eat anything (or if you have a dietary restriction).