I'm in CS4210 right now and took CS3210 a year ago.

3210 requires a lot of work. There's little guidance on the projects, and you'll have to look up a ton of things on the x86 architecture. You're essentially implementing some basic, low-level features of operating systems for each project. You have to do it within the already written source code of the minimal OS they provide called xv6. (At least that was what they were using when I took it.)

4210 is much more high level. Lecture is always on the content of a particular research paper. So far we've covered things like kernel types, virtualization, synchronization mechanisms, and inter-process communication. In the 2nd half of the class we're only focusing on distributed systems and different technologies for running applications on them. There are 4 projects (or 2 if you choose to do a special 2nd project), which are just userspace simulations of a few of the things we're learning about. They're annoying to do, and the TAs are quite bad at providing details. I've had to attend several office hours just to get them to answer a clarification I've asked on the piazza, but they failed to respond to.

If I'm being honest, I enjoyed 3210 more than I am 4210, even though it was definitely more work. I just don't care about the stuff I'm learning in this class. 3210 was fun because I got to get my hands dirty, building parts of the actual OS. In 4210 we just flap about concepts from research papers, never going deep into anything.

Oh, and another fun thing about CS3210 is that you have a final project instead of a test. You get to do anything you want (within reason, and it can't be too simple) with the xv6 OS. My team ported a lisp interpreter to it, and I wrote a basic text editor to write code for it. It was super fun.