I believe overdrive should automatically be enabled on any car. Overdrive allows a car to drive at fast speeds with low engine input (low revs) resulting in better gas mileage. The downside is that you lose torque. You should be able to turn it off for things like climbing mountains and using engine breaking to go down them. Unless you turn it off, it's already on.
How do I change out
There is no "switch". To cancel overdrive move the gear lever from "D" to "4".
The 2004 Toyota 4Runner runs on regular unleaded.
The 2007 Toyota 4Runner runs on regular unleaded.
The 2006 Toyota 4Runner runs on regular unleaded.
The 2012 Toyota 4Runner runs on regular unleaded.
The 2005 Toyota 4Runner runs on regular unleaded.
The 2008 Toyota 4Runner runs on regular unleaded.
The 2011 Toyota 4Runner runs on regular unleaded.
The 2014 Toyota 4Runner runs on regular unleaded.
The 2009 Toyota 4Runner runs on regular unleaded.
The 2010 Toyota 4Runner runs on regular unleaded.