Would it damage an engine further if you attempted to start a vehicle that has been sitting for 3-4 years? It worked perfectly fine before minus the AC.
Or should the fuel be drained and then attempt?
Lastly would leaving gas cap off and sitting for a month or so let the gas evaporate out?
Did the gas have Stabil added to it years ago? Did it have ethanol it it or non-oxygenated? How does the gas smell?
I’d probably siphon it out and add fresh non-oxy gas. Change the oil, then try to start it.
Did anyone put stabilizer in the fuel before the truck was parked?
If not the gas may have gone bad and the fuel system may be gummed up with varnish. The gasoline will not evaporate just by leaving the cap off. If you want to drain the tank you’ll have to disconnect a fuel line and drain it. The tank is only part of the problem, however, as the gasoline in the rest of the fuel system might also have become varnish.
Does it still smell like gasoline when you remove the filler cap?
Before you try to start the engine, remove the spark plugs and squirt a bit of oil into each cylinder. You don’t need a lot. Then turn the engine over by hand or using the starter BEFORE you put the spark plugs back in. This will coat the cylinder walls and rings with oil.
Then you can reinstall the spark plugs and try to start the engine. Cross your fingers. The 3-4 year old gas may prevent it from starting.
After you get it running it’s probably a good idea to change ALL of the fluids. The rubber belts are also suspect. The V-6 4Runner has a timing belt.
Unfortunately stabil was not added. It is likely ethanol fuel and is low.
There was never an intention to leave it but a flat tire, a nasty bees nest in running board, no ac in summer has let it sit.
No stabilizer and lots of air in the tank is the worst-case scenario. I’d drain as much gasoline as I could before doing anything else.
Even if you get it running the AC still won’t work without an infusion of cash.