1. His hardest decision was not to go abroad.
2. His hardest decision wasn't to go abroad.
3. His hardest decision was / not to go abroad.
4. His hardest decision was to not go abroad.
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#1 is a little difficult to comprehend. Sentence 1 has two meanings, right? I mean #2 and #3. Is Sentence 4 grammatical? Would you explain the differences among the sentences? Which expressions do we have to use? Thank you.