Just got back from Laos and went to both 4000 Islands and Nong Khaiw. Both were really nice. 4000 islands was very sleepy and laid back, could hire bikes and ride around the islands and also went on a day kayaking. Felt like the sort of place you could while away a week just lazing in a hammock and drinking pineapple shakes. Only problem is its a long way from Vientiane and Luang Prabang. We flew from Pakse to Vientiane to avoid the long bus ride. Nong Khaiw alternatively is only 3 hrs by bus or 6 hrs by slow boat from Luang Prabang. Also well worth a visit. Bus drops you off on one side of the river and you have to walk (or catch the overpriced tuk tuk) all the way to the other end of town and then cross the bridge to find most of the guesthouses. We went for a wander along the road to some caves and theres lots of day trips you can do (we did they 100 Waterfalls trek, the most expensive but it was really good, you are literally walking up waterfalls most of the way.)
Really enjoyed both places so given the amount of travelling involved in getting to 4000 Islands I'd recommend Nong Khiaw. And if you do go to Nong Khiaw I'd recommend the Lao BBq or the whole spicy fish at Yongkhoune Restaurant - so good we came back the next night for another round.