Ialready know how to play the ukulele, guitar, piano, and harmonica. i want to learn another instrument (preferably a stringed one), what instrument do you recommend.

It depends how much of a contrast you are looking for. If you want a more classical stringed instrument, I would reccommend either the violin, cello, viola, or bass.

As for a more typical country style stringed instrument similar to the guitar, I would suggest some more interesting instruments such as the banjo, mandolin, or veena.

The harp would also work, too!

The question is, have you ever played bass guitar? It is a very fun instrument to play and sounds amazing if you can play it right.