(like pkg2zip for extraction, or orbis-pub-gen for PS4 PKG creation) are open-source and hosted on GitHub. No legitimate developer would name their tool "PS3 to PS4 PKG Converter."
The only scenario where a converter might emerge is if someone creates a that translates PS3 bytecode to x86 ahead of time. This would require: ps3 to ps4 pkg converter
Refurbished PS3 consoles are inexpensive. Jailbreak a PS3 (CFW or HEN) to play backups directly on native hardware. This is the most compatible solution. (like pkg2zip for extraction, or orbis-pub-gen for PS4
Most online search results claiming to offer such a tool are: (like pkg2zip for extraction