Radial-velocity measurements of Kepler-454. Top left panel: HIRES (red circles) and HARPS-N (blue circles) radial velocities and the best-fit model (black line) with a long-term linear trend and three Keplerians corresponding to the transiting planet Kepler-454b (P = 10.57 d) and the two cold giant planets Kepler-454c (P = 524 d = 1.4 yr) and Kepler-454d (P ~ 4070 d = 11.1 yr). Top right panel: linear trend after removing the three Keplerian signals, which is caused by an additional fourth companion of yet unknown nature. Bottom panels: phase-folded radial-velocity signals of the cold giant planets Kepler-454c (left) and Kepler-454d (right). Note: orbital phases equal to 0 and 1 correspond to inferior conjunctions.