Play the instrument that you want to learn the most. Plain and simple. If you start to learn to play an instrument because someone says you need to, and have no real desire to play that instrument, chances are its going to be a dust collector.
By picking up an instrument you want to learn, chances are you will succeed much faster, practice more, and learn to love music and play another instrument down the road.
Please, dont be swayed by someone saying..."You have to learn this first." Makes no difference as long as you have the desire and are willing to practice. And by the way.... basses have 4,5,6,7,8,9,11,12 or 13 strings. Not just limited to 4 strings. If you do choose bass, I dont recommend learning on anything higher than a 5 string to start. The necks get VERY huge on these others, and good technique for a beginner would be kind of hard to do on some of these basses.