Will nkg iridium spark plugs be ok in a 1999 Toyota 4 runner 3.4 liter engine?

Maybe, but I would not use anything other than original equipment spark plugs in this engine. They are engineering specifically for the 4Runner and they work very well. I don't think you will notice any improvement with iridium plugs, and they may cause problems. They also cost a lot more than original equipment plugs. There are cases of non-original equipment plugs causing problems in these engines.