If the engine will not crank over, the most common problem is the starter. These will tend to go out at about 100,00-125,000 miles. Sometimes it is only the starter solenoid, but often the complete starter (including solenoid) needs to be replaced.Google+
Car Problem Reports
Toyota 4Runner Vehicle Won't Crank Due To Starter
Toyota 4Runner Problem
Average Mileage: 104,900 mi (66,700 mi - 141,000 mi)
Visitor, 2005 Toyota 4Runner, 66,700 mi
Intermittent starting. Vehicle would take several times to crank, no clicking sounds at all. Battery, ignition and alternator all okay. Replaced starter because of corroded contacts and corrosion on/in starter.
Visitor, 2005 Toyota 4Runner, 107,000 mi
Click, click, click... engine will not turn over. Starter failed/failing.
Visitor, 1996 Toyota 4Runner, 141,000 mi
The engine will start intermittently but the starter needs to be replaced.