"We hope it will pave the way for the crucial step of rehabilitating and reintegrating Maoist combatants".
The former fighters were confined to UN-supervised camps as part of the 2006 accord that followed the end of the conflict between Maoist fighters and the state.
In 2007, the United Nations Mission in Nepal (Unmin) completed a verification process and found that 2,973 were minors when the war ended and another 1,035 were not genuine combatants.
'Committed to peace'
The minors had been due for release soon afterwards, but the process was repeatedly delayed by disagreements between the Maoist party, now in opposition, and its political rivals.
Over the next month, all 4,008 will leave the camps, a move Chandra Prasad Khanal, the PLA spokesman, said would "send a message to the world that we are committed to peace".
"For us this is a sad moment because we are sending away our fellow fighters in the decade-long people's war. But we are taking this step in order to bring the peace process to a logical conclusion," Khanal said.
Media access to the camps is restricted, but in a briefing organised by the PLA, one former child soldier expressed his regret at leaving the camp.
The discharge of the former child soldiers will allow the Maoists to be removed from a UN list of organisations that use children in conflict.