If you have a homebrew-enabled console, here is the standard workflow for installing game packages:

You take a standard PS2 ISO of the game and use a conversion tool (like PS2 Classics GUI ) to wrap it into a .pkg file that the PS4 can recognize as a "PS2 Classic".

: To use a PS4 PKG of Guitar Hero 2, your console must be on a exploitable firmware (typically 9.00 or lower) to access the Package Installer Conversion Tools : Most "hot" PKGs are created using the PS2-FPKG tool

There is no official PlayStation 4 release for Guitar Hero II