The orthogonal arrays with mixed levels have become widely used in fractional factorial designs. It is highly desirable to know when such designs with resolution III or IV have clear two-factor interaction components (2fic’s). In this paper, we give a complete classification of the existence of clear 2fic’s in regular 2m4n designs with resolution III or IV. The necessary and sufficient conditions for a 2m4n design to have clear 2fic’s are given. Also, 2m4n designs of 32 runs with the most clear 2fic’s are given for n = 1,2.