In four days...

In four days, I will have an exam in higher mathematics. I am very afraid. =)

Есть три типа людей:  Те, которые способны к математике, и те которые не способны к ней.

After 4 days...

After 4 days, I will have an exam on higher mathematics. I am very afraid =)

"will" is used to talk about sudden future decisions

My cell phone is dying, I'll bring the cord with me.

Okay, bye!  I'll call you when I get home.

In this case, you communicate the futureness of this sentence with the time marker. 

The most usual way to say what you have here would be:

I have a math exam in four days.

present simple + time marker with future meaning.  Very common.  You don't always need "will" or "going to"