Non-Hazamable!

 If 2 people testify that Reuven killed Shimon and two other people claim that he didn't, how can we accept the latter testimony. It is עדות שאי אתה יכול להזימה [b/c they can't be punished if found to be זוממים for they didn't offer testimony that would incur a punishment] and thus shouldn't be valid??