How is the weather in L.A.?

1. It's sunny and hot this time of year.
2. It's sunny and hot this time of the year.
3. It's sunny and hot at this time of year.
4. It's sunny and hot at this time of the year.
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Which ones are grammatical? Do we have to use 'the'? What about the preposition?

All are correct, really. 1 and 2 seem more colloquial, but are not incorrect, per se. All are rather informal, colloquial. In 1 and 2 the preposition is implied, but it is necessary, stated or implied.