I've been playing bass for about four months now, and I thought about learning some major chords on guitar. Do you think it would help improve my overall musicianship?
Learning any other instrument will help broden your horizens, and therefore, make you a better musicians. You can learn something from every instrument. There is a thread about guitarist who started playing bass, and a lot of them said that their knowledge of guitar carried over to bass, and they learned a lot of stuff about guitar from playing bass. It certainly won't hurt to try playing guitar.

Bass is my third instrument, and I am learning a totally different side to music. I am learning how to play just enough to fit the song, and I'm learning a totally different type of theory than I would on my other two instruments.

If you are given the chance to learn any new instrument, jump on it.

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totally helps you. helped me lots. the variety of arpeggios you will have at your disposable triple and you konw just which one fits the song. plus it'l help you learn to hold double stops and triads and stuff better. plus you kinda figure out how bass and guitar interact in a song.