Lost remote for model 4640X how do I get replacement

The 4640 will work with any Roku remote you can buy anywhere. It can use the simple Basic Remote (IR only) or any of the Voice Remotes (WiFi Direct). Since it can use an IR remote, you can even find very inexpensive universal IR remotes (less than $10) and almost any store that sells remotes. Just make certain the packaging lists Roku players as a supported device.