Another simple thing to check: Is there a bluetooth mouse paired with the SP4? If there is, could it be turned on and rolling around in a bag somewhere? I've had similar symptoms because of just that.
Great guide. Follow to the letter and you'll be home free. Let me emphasize even further that the ribbon component of the home button is VERY fragile. Although my profile says I realised the procedure successfully, I damaged the replacement part at the crucial moment. Very frustrating to say the least. So be EXTREMELY careful when manipulating the home button. :)