is there a way to change the 4 buttons on the remote to other channels?
No, but there's a cult of Handheld Advertising Haters that surgically remove un-programmable buttons:
1) exacto knife
2) black tape
3) trash can/bio-hazard bin (to flip extracted buttons into)
Kind of a learning curve to endure, but by the 3rd Button Extraction Surgery, you'll be pretty good at it.
(Note to Roku Remote Advertisers: Those buttons you're paying for reside in the landfill where they are basically unclickable - have a nice day)