The Mishnah in tractate Pesachim gives the child's questions, and its list is not identical to the one recited today — the question about roasted meat belonged to a time when the Passover offering was still brought, and it dropped out. The reclining question took its place.
מְסֻבִּין (me-su-bin) and מְסֻבִּין (me-su-bin) describe eating propped on the left side, the posture of a free person at a Roman banquet. The fourth contrast is therefore not about food at all; it is about who you are while eating it, which is the point the whole evening is making.
The two dippings are the karpas into salt water early in the evening and the bitter herb into charoset later. The question notices a detail of the choreography, which is what a child at the table would in fact notice.
The Haggadah text on Sefaria carries a stage direction before this passage — remove the plate, pour the second cup, the child asks. It is instruction to the host, not words to be recited, and the scaffold dropped it rather than printing it as a line of the song.