Are you talking about the beeping that occurs when you switch to 4Lo without shifting the transmission to Neutral? That beep? Is the 4Lo light blinking or steady?
Shift to Neutral, switch to 4Lo does the beeping/blinking stop?
As the others have mentioned, it would appear that you aren't actually in 4Lo when you think you are. If it beeps at you, it means that it cannot engage. Put it into neutral at a stop and then engage it. Do the same to disengage.
Any time your 4runner beeps it means you're pi$$ing it off.
But it's good to see you're trying to use it. Take you're time getting into and out of 4Lo. Try to get in to it too quickly and you hear beeps. Try to get out of it too quickly and you'll hear some slight (VERY LOUD!) gear grinding.