Here is the completed passage with the appropriate options filled in:

The Internet is the world's largest computer network [1] with over two billion users worldwide and this number is increasing fast from year to year. Now, you can sit at your computer, choose a route and arrive at your destination - that is, the information you need. Today, from your computer, you can access a library in one country, talk to an expert on a certain topic in a second country and send your own information to someone doing research in a third. So, [2] we don’t know what will be next in the not-too-distant future. The Internet was born when scientists attempted to connect the American Defence Network to other satellite networks in order to exchange information. Soon, [3] scientific and educational institutions as well as research departments became interested in linking up, and, before long, the Internet had expanded into an international information and communication network. The Internet now reaches all the countries in the world. You can be connected to the Internet directly through your own computer or [4] you can hook up to an access system which allows you to use some of the Internet’s services.

Explanation:

1. with over two billion users worldwide fits logically, as it refers to the number of users on the Internet.

2. we don’t know what will be next is a reasonable continuation about the unpredictable future of the Internet.

3. scientific and educational institutions as well as research departments became interested makes sense as it describes who became involved in the development of the Internet.

4. you can hook up to an access system which allows you to use some of the Internet’s services fits as an alternative way to connect to the Internet, besides using a personal computer.