What can I do if my brother gave me a trailer four years ago without a title and now he is telling me he sold it even though I have been living in it?

You would have to sue him though giving over the title usually is how you give such a gift.

The general rule on transfer of items with title, of a certain value, of a certain term (time period) and always transfer of interest in real estate beyond certain (length of time) is that it has to be transferred with a writing. This rule is called the "Statue of Frauds." The details of how it applies and to what it applies varies by jurisdiction. You should consult with an attorney licensed in the jurisdiction the property is located.

I would suggest contacting a civil attorney with experience in contract and real estate law as soon as possible.