The primary mouse button would be the left-click.
Without it you wouldn't be able to open/run programs with your mouse. Also people use the left-click more than right.
You can make either button the primary mouse button, the default is the left button.
By going to;
Start-->Control Panel-->Printer and other Hardware-->Mouse
The mouse buttons are configured in the Mouse section of System Preferences .
Clicking an on-screen button, like an "OK" or "Cancel" button.
I have a Microsoft Wireless Laser Mouse 6000 and it has two side buttons along with the left and right click buttons.
On a Mac the Secondary Mouse Button is what is often called a Right Click - The primary button being the Left Click. When you click the Secondary/Right hand button you will see a menu of options appear from the mouse pointer. You can move your mouse down the list of options and click on an appropriate item. The contents of the menu will vary depending upon which application you are using when you click - which is why the menu is known as a Context menu. In the Mouse section of System Preferences you can configure the Secondary button to perform some task, other than activating the menu, like opening Spotlight to search for something.
A computer with an attached mouse allows you to move a cursor (often an arrow) around on the screen. By clicking the primary button of the mouse (Usually the leftmost button) you may select icons or open them. The mouse also aids in selecting multiple items and in completing other tasks that are difficult to do with only a keyboard.
secondary mouse button
The mouse button that is the most used is the 'left side' of the mouse.
Give the function of the right mouse button
LMB is the left mouse button
No no no no..it's double click your MOUSE.. hit the primary button twice smartly with your index finger..
there is no 4th button
That process is called the dragging process of a mouse