If you're in the US, you have either a 2.4L 22R-E inline-4 or a 3.0L 3VZ-E V-6.
The V6 4Runners had a small "V6" badge in the lower, driver's side corner of the front grille and on the lower, passenger side of the rear tailgate.
Also, when you look in the engine compartment, the 2.4 has one engine head with four spark plugs in a row running down the driver's side.
The V-6 has two engine heads with 3 spark plug wires going to each head.
The 3.0 V-6 is a larger engine with six cylinders instead of four, and the six cylinders are arranged in a V-configuration to shorten the length of the block. The 2.4L has four cylinders, all in a line. The 3.0 was rated at 150 horsepower and the 2.4 was rated at 116 hp.
Here are the fuel economy estimates:
2wd, 3.0, 4-speed auto 15 city, 19 highway
4wd, 2.4, 4-speed auto 16 city, 18 highway
4wd, 2.4, 5-speed manual 17 city, 20 highway
4wd, 3.0 4-speed auto 12 city, 15 highway
4wd, 3.0 5-speed manual 13 city, 17 highway
www.fueleconomy.gov