4qts seems abit low however he would not be able to put 4 qts in unless he drained the oil pan. Or he had a really bad leak.

Auto trans fluid is Red. Unless heavily burned which would be brownish and smell really bad. Not to mention the trans holds a lot of oil.

The drain plug is exactly where you described it and if the skid plates are like the 4runner their should be a hole in the skid plate for the drain.

If he did happen to drain all the trans fluid you should be able to fill it back up and be ok.

To check the autotrans fluid pull the tranny dipstick while the engine is running and warm. The vehicle should be level.

It sounds like he drained the right fluid and might have another problem but to be safe he should check the tranny fluid before attempting to put it in gear again.