Again, the idiom is 'one conflict of interest', 'two conflicts of interest'. Googling will give you almost 4 million hits on this form, and only 185,000 hits on the incorrect 'interests'.
Therefore, only (1) and (4) are right. They are all easy to understand, but the others reveal shoddy grammar, and the writer would be exposed as a simpleton. A 'conflict of interest' is the holding of opposing interests (either benefits or liabilities) within or between parties to an agreement, condition, or other situation.