Should start a job with 4 months remaining for cat or prepare for cat ?
It depends on how serious you are for CAT. If you think you can make it in 4 months, prepare well, which should not be a problem as 4 months are perfect if you put your heart and soul in it, then let the job be for now. And start preparing well for CAT, covering every direction. Work on your overall growth, join a Toastmasters club, or play regular sports. Complement your preparation with one other activity that will help your personal growth. Again, reiterating, only if you are serious about CAT. Otherwise, if you just wish to test the waters before the dive, take that job and start preparing from Jan, slowly and steadily. That will help you better.
In all honesty, I don't think you should take a job. It will be a big mental drain for you, regardless of what line of work you get into. My suggestion is that once you've taken CAT, depending on how you think you have performed, start looking for jobs. If you think you'll get calls from at least 3-4 elite institutes, then just wait it out. Or else, start hunting for a job in December and by January, get into a full-time job. This way, your career gap will not be egregious.
4 months is just about enough to prepare for CAT. You will need to invest more time in July, August, and mid-September on concepts. September 15th onwards, switch to mocks and sectional tests. Also, take as many exams as you can to increase the probability of getting into a top B-school in 2020. All the best!