maxton releasing GameArchives ArchiveExplorer (latest version HERE) with PS4 PFS support followed by MakePFS to build PS4 PlayStation File System images, well today he's committed a new project to Github called LibOrbisPkg which aims to be a library for PS4 PKGs!
Download: LibOrbisPkg-master.zip / GIT
According to the changelog, the latest revision includes a fix for GP4 reading/writing that no longer uses field layouts for PKG structs.
If any PS4 developers wish to lend a hand feel free to assist in the LibOrbisPkg development, and as always we'll add updates here when they become available!
Cheers to @Wultra for the heads-up of the news on Twitter!