I took a year off between 3rd and 4th year. I did a clinical research fellowship and would absolutely do it all over again. I had an incredible mentor who was extremely prolific. He had a ton of ideas and allowed me a lot of freedom to develop my own projects. I got a bunch of presentations out of the year and even a few publications. And, when it came time for applying for residency, my mentor went to bat for me. When I didn't get an interview at a program I was really interested in, he made a phone call for me. On every single interview I went on my research year was discussed. And, I wound up matching at the institution my mentor works at (not my home institution). If you have any specific questions about taking a year off, feel free to PM me.