This Thor Ace will be a great match for the XTRA 4 pack. XTRSP4.**Very Important -These pads are specifically made to fit flat landing feet (please see photos below), they will likely not work if your landing feet are bent or have become "dished." (as seen in the comparison photo). Prior to placing your order please visually inspect all of your landing feet to ensure they are flat. If you are unsure please feel free to send us a photo.