I took LS4 with Chen (it didn't give me the option to choose that class on the drop-down menu).
As a class, LS4 is hard. There are concepts to be learned and memorized...and you also have to learn to apply those concepts and do problem-solving. It involves time and practice, but it's not impossible. You can definitely get an A with the right amount and right kind of studying.
As for Chen....as a lecturer, she's truly average. Her class would sometimes fly by and other times it would drag, depending on what subject we were discussing. On the positive side, as someone mentioned below me, she is very welcoming to questions in class, and you should take the opportunity to ask her if you do have a question. On the negative side, she's not the most exciting of lecturers. But she does seem at ease in front of the class, so she's not horrible either.
The discussion sections are great because only 5 out of your 10 quizzes count towards your grade, and it really is an opportunity to learn. Though this does depend on your luck of the draw with your TA. All 3 of the fall quarter TAs were great in my opinion so we lucked out here.
I went to Chen's office hours and found them to be pretty useless. Just full of a bunch of brown nosers hoping for a letter of rec, asking inanely detailed questions :x
Her practice tests were very helpful, but do not be fooled. They are usually much shorter than the actual exams, and the difficulty varies. Her first midterm and her final were very, very, very, very long. They were difficult, but not impossible...but the length was killer. As the person below me mentioned, practice by doing the old tests she gives you, but try to twist the problems in as many ways as you can so that you're mentally prepared for the curveballs she may through at you.
Best way to prepare? Stay on top of your problems. TRY to keep up with the chapters as she's lecturing so that you can ask your TA if you run into any problems early on. Do all the assigned problems + more at the end of the chapters and do any practice problems your TA gives you in discussion. This class is 90% concept-based problems, and 10% just memorization. So make sure you practice.
Overall, I'd recommend Chen for LS4. She's not amazing, but she's nothing to stay away from either. As long as you stay on top of it, you'll be fine.