Is it safe to use an "18v ~ 20v" power brick/adapter for a device that originally had an 18v adapter?
The power supply cannot adjust itself automatically to what the device expects. It can adjust the output voltage to try to compensate for the voltage drop on the wires based on current draw though, but the original is only rated 2.5A and the replacement is rated up to 4A. Get a proper 18V replacement, with 2.5A or more current rating.