Welcome to beatthegmat! I think you have the ideal amount of time in your hands to prepare for the GMAT. 4 months should be good enough if you're able to maintain the momentum throughout this period.
I'd advise that firstly you understand the ins and outs about the pattern of the GMAT .After you've become familiar with the pattern of the GMAT, you may wanna download GMATPREP software from
www.mba.com and take one exam from there. A GMATPREP will help you give a rough estimate about your current competency level. Though it may not exactly reflect how good you're at Verbal and Quant,but it'd surely give you a rough estimate about where you stand now and what kind of questions appear on the exam.
After that, you may make a study plan for yourself. Focus on one area first say Sentence Correction and master it well. One of the mistakes which people make is that they study superficially without understanding the concepts(and if you go through my debrief,you'll realize that even I was a victim of this). You need to make sure that you analyze the questions really well and should have definite takeaways from them. Regarding the books, you may wanna buy MGMAT SC Guide or Aristotle SC Grail for Sentence Correction. For CR, Powerscore CR Bible is the best book. For RC, you may wanna go for RC99 which is an amazing source for practicing GMAT type passages.For Maths, MGMAT Maths Guides or Veritas Maths Guides are good.
Keep participating in the forums here as that will help you maintain consistency.
