Hello Saurabh,
First & Foremost, let me congratulate you--NOT many make this sort of an effort to plan out the career path even after passing out--let alone even before passing out.
Adding to what Ankit & Archna mentioned & to answer your query reg concentrating on CAT, etc, pl note that concentrating on completing your BTech is your first priority--since without this degree there won't be the next step.
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, pl forget for a moment about CAT, etc]:
1] Stop with BTech & join a job...obviously will have to be a technical line.
2] Do MTech & go into the technical line
3] Do MBA & 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-does not necessarily have to be parents, mind you. 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 suggest pl don't tax yourself too much. Like they say: cross the bridge when it comes.
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 approx 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, pl 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 jobs roles. At the end of the day, any organization is a group of people which needs to be guided/controlled/handled & THIS REQUIRES management skills.
So, there's NO ESCAPE from management, whether one likes it or not.
Hope this helps you to make a WELL-INFORMED DECISION, rather than confusing further :-)
Rgds,
TS