It's a very common number in Chinese and is used in many different contexts.
Here are some examples:
* 四個 (sì ge) - four (things)
* 四月 (sì yuè) - April
* 四點 (sì diǎn) - four o'clock
* 四千 (sì qiān) - four thousand
Keep in mind that the pronunciation of "si" might vary slightly depending on the tone and the context.