It has been years since+Past Tense or Present Perfect

[Brahman]

Dear Sir,

I have four sentences in mind:

1. It is years since I swam.
2. It is years since I have swum.
3. It has been years since I swam.
4. It has been years since I have swum.

I feel, sentences 1, 2, 3 are correct. Personally, I should, however, say only sentence 2. Could you please tell me whether I am right?

[Alan]

1 or 3. It means '...since I last swam' - a definitively past event, although present tense/present perfect in the main clause is optional.