Hi!

As someone who is taking a Calculus class now, it is really important that you make sure your Algebra and Trig skills are good-to-go. 

One thing I recommend doing is skimming through the Algebra Basics Topics. This will start with the very basics of negative numbers, and ending with Quadratic Equations. Depending on how these topics are for you, you could choose where to go from there. If you are struggling with these, start with Algebra 1. But, if you are finding all these topics really easy, you could try moving on to Algebra 2. Once you finish this, take your time going through the Trigonometry and Precalculus courses. It is crucial that you really know these topics before you move onto Calculus. 

If you are motivated and devote time to learning these math topics, I think you could do it in four months. It would just require a lot of discipline and self-motivation. Also, if you are finding it hard to figure out Trig and Precalculus, it could always help to take a class to really build that foundation.