If you’re a musical champ, you can do with a 4 fret. The point of the pocket guitar is to improve dexterity. If you’ve already got the hang of the strings and frets, then, you don’t need a 6 fretboard because you have a pretty good memory of the acoustic. With beginners, you will need to adjust to both the instruments. Although the pocket guitar is like an acoustic/electric, it is much smaller in size. To really get hands-on practice on both; without having to learn new skills every time you switch from pocket to acoustic, a 6 fretboard makes sense to invest in.