Can we create multiple users in one login in SQL Server 2008?
If you are using SQL Authentication, then the short answer to your first question (do I have to create 4 logins, one for each user?) is, Yes. You do.

It sounds like you're looking to group individual users under a single entity.

If you are able to use Windows Authentication then you can create an Active Directory Group, let's call it GP-DatabaseUsers. Add the Windows logins as members of that group in AD, and in SQL add the group as a user and attribute the desired credentials to that group. You can still track individual login usage, but you'll be controlling security of a single entity.