Sorry to hear about that - it's horrible when that happens!
You can install PT4 again from the
PT4 Download Page and you can access your license from
My Account > Registration Codes at the top of the page. You don't need to pay again - your old computer doesn't count against you.
If you have a backup of the settings and databases from your old computer you can restore them with File > Restore, or if you have the old hand history files you can import them with Get Hands From Disk.
If you don't have either of these, you can make backups using File > Backup. We recommend making regular backups and storing them off your main computer, as you would for any important data.