Plot Twist cost
so if I have 4 resource cards in play then anytime during the game I can play plot twist that costs from 0 to 4 threshold and I can play as many cards as I want? for example I can play 17 plot twist cards that have a cost in the upper left corner of 1 on each card even though I only have 4 resource cards in the resource area.

Thanks!

That would be correct. Only characters and equipment cost "resource points" so they are limited to the 4 point total in your example. Plot Twists and Locations can be played as much as you'd like so long as the single card threshold cost is covered by the total number of resources.