Best course of action...
1. Install rekordbox 6 on that old computer.
2. Upgrade the database from 4.5 to 6.
3. Backup the library (File > Library > Backup Library), include the music.
4. On the new computer, install rekordbox 6.
5. Restore the library (File > Library > Restore Library).
No need to worry about relocation or iTunes; it will do it all when you include the music as part of the backup / restore process.