The US pronunciation of 'liberty' is /ˈlɪb·ər·t̬i/
The British pronunciation /ˈlɪb·ə·t̬i/
Nobody would ever say /'lɪbərətɪ/ as four syllables. It is always a three-syllable word, whichever pronunciation you use.
If you are not sure how to pronounce a word, all you need to do is check any good online dictionary. The more words you hear pronounced correctly, the easier it will become for you. For example, if you know how to pronounce 'liberty', this will help you to pronounce similar words, such as 'property', correctly.