The answer is: Sentence 4.
Sentence 4 has a subject-verb agreement error. The subject in that sentence is plural ("they") while the verb is conjugated in the third person singular ("decides").
According to the Subject-verb agreement rule, subjects and verbs must agree with one another in number. Thus, a singular subject takes a singular verb while a plural subject takes a plural verb.
The correct version of sentence 4 is the following: "Ultimately, they decided to study fossilization." The reason behind this is that the subject is plural so the verb must also be conjugated in the plural form.