You got the right idea. Just push that to the side and push the 1.5 side up the smaller pipe then slide it back down over the four inch. It’ll move. If its truly unable to swing side to side you may need an additional 1.5 Fernco cplg. cut the pipe a foot above. Slip the ferncos over everything as described above and put it in place and slide everything back over the existing pipes. You may want to mark the half the length of the slip cplg so you get it even on both sides.