Fourth hand opens the bidding if the number of high card points and the number of spades total 15 or more. Reasoning is that high cards are probably evenly divided, so 4^th hand opens only with a spade suit (highest ranking) to compete.

II.

To count a hand using the Rule of 20:  If you have 10 high card points and two 5-card suits (10 cards), add them together for a total of 20 and you should open the hand.