I expect delivery of new hot tub. I have in place a 6 inch thick concrete reinforced pad which served as foundation base of a previous 500 gallon wooden hot tub.. Ontop of that 6 inchpier blocks upon which rest 4 x4's crossed by another layer of 5 4 x4 's with spaces between. They were added to raise level of new hot tub. Question-- Do I need to put on top of 4 x 4 's a layer of decking planks for a 79 x 79 hot tub? Can the new hot tub rest on spaced 4 x4 's?

I am assuming the space between each 4x4 is more than an inch or 2.. So I would recommend it yes.
It will help disperse the weight more evenly. If you have larger gaps under the hot tub, you will get more stress on the parts that are supported. I could see this causing problems. Remember 1 gal = 8 lbs. 500 gallons = 4000 lbs. that's 2 tons just for the water.