I'm trying to complete all the skill challenges for instruments for my sim but I don't know how to learn the 4th master tracks legitly. You get one for reaching level 10 on a skill and two more for completing challenges but how do you get the last one?

I can use MC but it glitches my sim by making sheets they've learned readable again so they actually learn duplicates of a song.

So does anyone not have any idea? I've checked some xml files and it seems the 4th master tracks can be learned from opportunities but other than that, I couldn't find out anything else.

I would say the 20 or so people (including you and me) who have viewed this thread thus far have no idea. But in addition to being a holiday over here in the US, the forum boards on this site don't usually move that quickly. It's really more appropriate to wait at least 3 days before bumping an unanswered question if you could, please. 

Oh, sorry about that. I'll be more careful next time.

Info here: https://sims.fandom.com/wiki/Instrument_skill

Looks like 3 of the master tracks relate to the challenges, the fourth (the one you are looking for I presume), is achieved through a "certain" opportunity but it doesn't say which one. And since opportunites are random, I'm not sure how you would complete this without using MC to force opportunities.
One suggestion would be to make sure your Sim has only the guitar skill so that you only get guitar related opportunities but that is still random.

Thank you. I tried MC and even Debug Enabler to force the opportunities but none of the skill opportunities or career opportunities give out the 4th master track as a reward. From what I know, the last track should be a music sheet.

It could in be the repeatable opportunities as a random reward with very low chance but I still can't confirm it.

Under Learn that Tune in the Guitar Opportunity section
Learn "Master Track: Walk the Path" composition and go to work.The record label wants you to learn a new song for an upcoming performance. You've already been given the sheet music; you just need to spend some time to learn it! The record label will pay you for your efforts and your job performance will improve as well.
Notes: This is a special opportunity that allows Sims to learn the 4th guitar master track: "Walk the Path." As soon as the opportunity is received, the master track sheet music will appear in the Sim's inventory. There is no other way to obtain this sheet music other than from receiving this opportunity. However, the opportunity itself only exists in the game files, thus making it impossible to actually learn the final master track.
https://sims.fandom.com/wiki/Opportunity/Music

"However, the opportunity itself only exists in the game files, thus making it impossible to actually learn the final master track."

Well, no wonder even Debug Enabler couldn't give me the opportunity. Thank you! At least now I know I was searching for the impossible. It's too bad the other instruments don't have their opportunity pages filled out. I'll try looking into the opportunity xml files and see what I can find for the others.