Employer threatening to sign offer letter is it valid

I am working at a company since the last 4 months. Initially, they told that I was under probation period verbally and I trusted them and started working. Every time I asked the manager to provide me an offer letter, he used to come up with different excuses and said he would give it later. Salary was credited for these 4 months. Since I had no other option, I believed him and worked for 4 months. In the last week, he suddenly gave the offer letter and I saw that the clauses mentioned were unclear. When I questioned them to show me the company policy, they refused saying that the policy is still under preparation. When I disagreed to sign it, they tried to convince me by saying that these policies don't matter and that I was thinking too much. Also, it was clearly written in a clause that the letter should be signed on or before date of joining, which is 4 months ago. He also threatened me saying that they can file a suit against me for taking salary. What should I do?