The correct word is preventive, not preventative — three, not four, syllables. You create the adjective from the noun, so prevention becomes preventive. If preventation were a word, you could justify using preventative as the adjective form. But preventation is not a word, and there is consequently no logical basis for adding the extraneous ta syllable to preventive.
Unfortunately, preventative is spreading like a noxious weed — especially in academic and scientific writing, but also in everyday expressions like preventative measures — and is in danger of becoming a grammatically acceptable variant of preventive. We must not allow that to happen on our watch.