I looked up info on over 60 rads programs (didn't apply to that many though). None of them required 4 letters. Most just said "3 letters of recommendation," but it's usually understood that you can send another. Per Dr. TLM on Aunt Minnie, the 4th letter should be a research letter if you have one. Otherwise, the people who review your application only want to read 3. Sending a 4th clinical letter is not necessarily bad (in my understanding), it's just that they might not read it. I'd offer to send you the info I put together, but I just did it as I went through adding programs to ERAS, and didn't keep a record.
Prelim programs understand that you are one and done. It shouldn't be an issue if a letter says you would be a good radiologist. However, you might want to consider which specialty each of the letters is coming from. Medicine and surgery letters are the best if the content is otherwise comparable. An awesome letter is still an awesome letter, so use judgement and common sense.
A radiologist letter is probably less helpful for prelim, since most of us have limited experience doing meaningful work in the rads department. I'm really really good at holding a chair down - they never escape and I have unparalleled skill in finding an extra one if needed. Other than that and a few "How did you know that!" comments in anatomy pimping sessions, I didn't do much that was amazing in the reading room. I definitely used my other letters for prelim and just sent one rads letter to rads programs.