Hello David Gorin,
Thanks for reaching out to Microsoft Community.
Normally, the date of purchase can be found on the receipts. Also, the starting date of the standard limited warranty could also be the date of purchase of your device.
But I regret to inform you that Surface Pro 4 has already been officially discontinued and no longer has replacement available for hardware related problem. With being said, it is best to visit our official website: Surface.com to check for updated hardware specifications which are being used nowadays. You can also check your trade-in options through Microsoft Trade In & Recycling Program - Microsoft Store.
Otherwise, don't hesitate to respond to this thread if there’s anything else that needs to be addressed.
Regards,
StevenK