Must totally agree with the wisdom of recommending a Galaxy Note Tablet (10 or 12 either one would probably do). College note taking is going to be pretty demanding and even as big as the Note 4 is, its not a tablet and not a full sized notepad. And the S4 is still a respectable phone. I just upgraded from one and my son took it over. Let me suggest a compromise solution.
First, yes look at the Galaxy Note Tablet. It's a great tool for note taking. I use it for work meetings and have lots of notes.
For a nice add on to your phone, go to Amazon and grab one of these:
The Joy Factory Pinpoint Precision Active Stylus with Ultra-Slim Tip (BCU201) for iPad, iPad Air, Mini, Samsung Galaxy Tab, Note, HP Elite, Google Nexus and related tablets
It is no S-Pen. But it works respectably well with LectureNotes or the S-Memo program you have on the S-4. Great for out and about quick note taking.
So now you'll have a full size tablet for class, and an adequate solution for out and about when you really need to write something quick (assuming of course that you carry the "pen" with you since the S4 doesn't have any storage compartment.
Here is another stylus alternative for the S4 that is much cheaper and not bad (and more compact), but not near as good as the powered one above:
iKross Galaxy Touch Screen Stylus Pen for Samsung Galaxy S5 S 5, Galaxy S IV / S4 GT-I9500, Galaxy S3 / S III - White
Again, these styluses are not like the S-Pen and in no way a replacement for note taking on a notepad or tablet. Just a way to get limited writing ability on your S4 so you can get the tablet.
Now of course if you could afford to upgrade your phone AND get a tablet, well that would be great. But my memory of college was one of not so available funds
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There are several Youtube tutorials by college students showing some pretty fancy note taking using their Galaxy tablets. Take a look around. LectureNotes and the delivered S-Note program are great. Another one to consider is Kno, which was specifically designed for college use. Can get textebooks through them as well as import PDFs for annotation and collaborate with other students that have it.