Ques. Should I buy a 3D tv(Sony 42 inches) or a 4k tv(Samsung 40 inches)?
Ans. Sadly we would recommend you neither of those. The hardline is that their is no substantial content available at the moment for 3d or 4k broadcast in India. Any of these will just become a toy to flaunt to people after a few days of usage after which you will find no use for the 4k or the 3d technology. Tata Sky a few weeks ago did launch a 4k compatible set top box, but what they forgot was to accompany it with a 4k channel. With no 4k content this box would be as useless as a simple non HD set top box. You will also have to shell out a premium for the 4K set top box and the 4K TV.
Also we have found the LED/LCD tvs of premium brands like sony/ samsung aren't so reliable for the premium price they demand, and may not even last till you get the 4K or 3D content readily available in India.
Our advice to you would be to buy a HDTV for now and after some years when the 4K or 3D content is available you can get a 4K or a 3D tv then, also the price of the 4K TV would be much less than what the price is now.
As of now just HD is suffice.