depending on when your friends arrive, they could go for lunch, drive over to the Marriott at Kalapaki beach and enjoy the scenery there, go to Costco and stock up on groceries, check out the Kauai Museum in Lihue (great place) drive up to the waterfalls from Kapaa, drive to the coconut coast and enjoy the beach. all within short distance from airport Easy to use up 4 hours I think.

Try Costco (if you are a member) costcotravel.com. they have the best rates for cars and they also waive the extra driver fee of $10.00 a DAY in Hawaii. Since you have 2 couples I am assuming you will want to have an extra driver and in Hawaii, you pay even if it is your spouse.