Yes the N4A642GKB is compatible with the END4X42L21A, but only when paired with an appropriate furnace will it be an AHRI-rated system. Without the furnace, it will not perform nearly as well. We have several systems with the two units already matched with appropriate furnaces on our site; use our search bar to find "N4A642GKB / END4X42L21A" and full furnace and AC systems containing the two units will show in the search results.