Hi,
Here’s a link to the ifixit [guide|103404] guide, which should help you to open the phone so that access to the power button is possible.
If the problem is actually with the power button actuator and not a problem on the systemboard connected to the power button, then here’s a link to just one [https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=moto+x4+power+button+&_sacat=15032&LH_TitleDesc=0&_sop=12&_fspt=1&_osacat=15032&_odkw=moto+x4+power+volume+button+flex|supplier] of some power button parts.
It is only shown to give an idea of the cost of the part and what they look like. There may be other suppliers that suit you better. Just search for ''Moto X4 parts'' to get results.