Here are the words with the main stressed syllable placed differently in each group:

1. **necessity** (stress is on the second syllable: ne-**CESS**-i-ty, while the others have stress on the third syllable)
2. **cutlery** (stress is on the first syllable: **CUT**-lery, while the others have stress on the second syllable)
3. **explain** (stress is on the second syllable: ex-**PLAIN**, while the others have stress on the first syllable)
4. **pagoda** (stress is on the second syllable: pa-**GOD**-a, while the others have stress on the first syllable)

Thus, the differentiated words are:
1. necessity
2. cutlery
3. explain
4. pagoda