I am in my Second year of physics right now and I am just curious what the major changes from the first 2 years of math and physics take place in the third and fourth years. What should I expect? How can I prepare for these years and do well?
That depends on the school. My university scheduled the 2nd and 3rd year physics courses so you could "double up" on them, and complete the first three years of the major in two. I chose this option, so my first two years covered the standard undergraduate courses, and my last two years were full of special topics and graduate courses.
The point is, you should tell us what topics you have completed, and which ones you will take in the next two years. Then we can tell you what you need to brush up on to do well.
My first two years were challenging but not hard to manage, and I could easily juggle 16-18 credits (18 is the max at my school before you have to start paying extra). Now I'm at 16 credits with my first round of 300-levels, and attempting to self-teach intro chemistry for credit. I'm drowning in work and sometimes feel as though I don't understand key concepts. I'm still doing reasonably well in classes, but for me personally I've never gotten a B+ in anything and don't want to start this semester.
My advice would be to brush up on all the math and physics you've learned during the summer between sophomore and junior years. You're going to need all those concepts to come back quickly, otherwise you could be stuck on homework for hours. Also if grades as well as fully understanding your classes is a big priority for you, take less classes as you'll be able to devote more quality time to the important ones.