I am newbee to arduino. I want to ask, how much time arduino takes to read a 4 bit string?
Reading a 4 bit string does not make sense. A 4 character string, maybe.
Reading from an SD card will take 2 amounts of time, depending on whether the data is in the buffer fetched from the card last time, or not.
Reading from the serial port will depend on the baud rate, whatever else the Arduino is doing, and how fast the other end is sending data.
There is no single answer to your highly ambiguous question.
In general, it really doesn't matter, either.