It is possible to place electrophiles in position 4 of the pyridine through the formation of N-oxides.
I detail the three stages of the reaction:
1. Formation of pyridine N-oxide
2. Electrophilic substitution at position 4
3. Reduction of pyridine N-oxide
The release of the oxygen couple makes the reaction work under milder conditions, not requiring such drastic conditions as in pyridine. The electrophile can also enter position 2, although in a minority due to steric hindrance.
