Ashish, the '4 stroke' means the following four things happening in a serial order:
1) engine sucks air and petrol vapor, when piston moves outward in the engine cylinder cavity
2) piston moves inwards, thus engine compresses the air+petrol
3) spark is ignited and petrol burns, pushing the piston outwards forcefully
4) piston moves inwards, pushing the smoke outside.
5) .... steps 1 to 4 repeat, till the time engine is On.
As on date, all cars and most of bikes have 4 stroke engines. The good old Padmini, or the Bullet also had 4 stroke engines.
Such an engine, or any other type of engine has certain volume of the Cylinder - the 0.6 L is about 600 ml, which is the volume of the cylinder, in which the piston moves. So note, our old TVS-50 had only 50 Cc (i.e. 50 ml) cylinder volume - so less than one-third of a coffee cup!! (yes, the 'petrol' thing is that powerful when it burns!).
The engines produce power, i.e. capacity to drive its output shaft, to which gears, etc. are attached. It is measured in 'british horse power' - the bhp thing. One bhp is 735 (or 746 - I always confuse) watts. Bullet had about 18 hp, i.e. about 13 kilo Watts. Our mixers in kitchen produce only about 500 W mechanical power. Scoda Octavia car produces about 145 bhp. So its engine is about 4 times more powerful.
Power does not necessarily always mean 'pickup'. Some engines takeoff very slowly - tractors / trucks are best examples. (So please don't think Nano has only 1/4th pickup than scoda octavia).
Hope this settles most of your doubts.
New Tata Nano with EasyShift is available at Rs. 2 lac (Ex-showroom, Delhi), and the final cost depends on the valuation of your car. So, first get the valuation of your car from authorized Tata dealer, so he can give you a final quote for the new car.