Get to know month wise pregnancy guide which describes the various stages of pregnancy so you get to know what's all happening in the 4th month of your pregnancy. Do visit the link to read more.
I am in the 4th Month of my pregnancy what is that I should be knowing ?
That asking strangers that have little or no qualifications that you can check is not your best resource. With that said, you should talk to your doctor. Buy or check out from the library one or more of the many books on the subject, such as "What to Expect When Expecting". And/or get one of the many apps for your phone. Some tell you what to expect each week.
So many things. We advise you to read books and to ask the gynecologist about all.