I just finished watching OVA 3.
I enjoyed Sonic's voice. (He sang at the ending. Same as Saitama sang on ova 1, and Genos sang on the Ova 2)
I believe the OVA 4 will be sang by Bang, was it? Sorry I forgot the name. The old dude from S-Class.
Also, I forgot who is that person who's asking for the title of the endings of the OVA... The title is "One Punch Marching Song" I believe hue.
Will update soon.