According to the TOLOnews broadcaster, local officials said that the bomb detonated near the offices of the Independent Election Commission in the provincial capital Jalalabad at around 11:30 a.m. local time [07:00 GMT].
A spokesman for Nangarhar's governor said that one of the four injured remained in a critical condition.
The attack happened during protests of supporters of a candidate for parliament Javed Zaman Ghamsharik, who was rejected by the Election Commission and barred for allegedly possessing illegal arms and for having ties with illicit land deals. The protesters might have been rallying outside the offices when the explosive device detonated, according to the outlet.
READ MORE: Explosion Hits City of Jalalabad in Eastern Afghanistan, 15 Killed — Reports
The Afghan people have been suffering from a long-standing armed conflict between the government and a number of terrorist groups. The Nangarhar province and its capital Jalalabad, in particular, have seen a series of attacks and explosions over the past few months, with over 30 people killed since the beginning of July.