If you plan to leave it unattended, figure out a way to lock it. At least cable lock it to the seat track or something. You can lock just the lower recover if that's all the space you get.
You don't want to be "that guy" that gets your AR stolen. Very bad press and some states you are liable for an unlocked firearm.
BTW T4Rs are easy to break into. There is no factory glass breakage sensor either.
There are racks that attach to the ceiling. Not sure if the locks or how secure it is.
interior rifle rack mount - Toyota 4Runner Forum - Largest 4Runner Forum
Then there is the bed vault. Nice but takes up space.
You could fold down the seat and hide it on the floor.
