First and foremost, let me congratulate you—not many make this sort of effort to plan out a career path even after passing out, let alone before passing out. Adding to what Ankit and Archna mentioned and to answer your query regarding concentrating on CAT, etc., please note that concentrating on completing your BTech is your first priority—since without this degree, there won't be a next step.
Career Path Options
Coming to the dilemma of which direction to go from here, here is some food for thought. To summarize, you have the following paths you wish to evaluate. To begin with, please forget for a moment about CAT, etc.:
1. Stop with BTech and join a job…obviously will have to be a technical line.
2. Do MTech and go into the technical line.
3. Do MBA and get into the management line.
So the "First Choice" you need to make is whether you want to go into the technical or management line. You will be the best judge for this decision...or take the help of those who know you best—it does not necessarily have to be parents. For this step to be done in a well-informed way, you may need to consider your basic temperament, aptitude, attitude, interests, etc.
Once this choice is made, indirectly, you have your answer whether to focus on CAT or MTech or stop with BTech. Assuming you decide to go for the management line, then comes your dilemma again on which field to choose—I think there's time for this step. So, please don't tax yourself too much. Like they say: cross the bridge when it comes.
Does an MBA Nullify BTech?
Now coming to your core query: "Is it really true that doing MBA after BTech nullifies our 4 years spent in the BTech?" In a way, this input will be needed for your Step 1 decision-making process. In general, a BTech graduate is one of those who has a higher IQ than many other lines/subjects of study—basically since the analytical thinking that this degree needs is higher. Hence, such an IQ background will only become an added advantage to any other avocation pursued. Since your subject in BTech is CS, if you do MBA, you could fit into any of the IT companies in management roles—since you already have the approximate domain knowledge. And this aspect doesn't come into the picture if you decide to pursue the technical line.
Another point that will have a bearing later on in your career: Irrespective of your choice being Technical OR Management line, please note that invariably EVERY TECHNICAL PERSON WILL HAVE TO GET INTO THE MANAGEMENT mental frame of mind as his/her career progresses—including Design/R&D job roles. At the end of the day, any organization is a group of people that needs to be guided, controlled, and handled—and THIS REQUIRES management skills. So, there's NO ESCAPE from management, whether one likes it or not.
Hope this helps you make a well-informed decision, rather than confusing you further.
Regards,
TS