Answer : 4. "4"
Explanation :

1. When the Tab key is pressed, 4 space characters are inserted.

2. The following are the tasks performed by the keyboard-

- With its help, alpha-numerical data is easily input into the computer.

- In the absence of a mouse, many of the tasks done by the mouse can be done.

- Through this, commands are given to the computer.

- By using the combination of buttons (shortcut keys) present in it, many operations can be performed, which saves a lot of time.

- Apart from this, tasks like playing games, controlling music etc. can be done.