Aloha!
I use to work for a Hawaiian Tour Company..
4 bedrooms can be very tough to find..
so when you find it...don't wait too long to decide
Definately 2 cars ....maybe a mini van and then at least a midsize
Also many units can be on the 3rd story with no elevator, so check that out
In the Poipu area I do love Kiahuna Plantation, BUT they are not air conditioned UNLESS they have begun updating the units WONDERFUL grounds and quiet beach ..or at least it was
As you've probably discovered in your research ....
Poipu is the busiest and constantly under some sort of construction...they have a Home Depot on the island

HOWEVER the weather is better, up towards Hanalei you will get more rain. It's so green for a reason!!!!
Weather is totally unpredicatable, but usually it's liquid sunshine as it's a burst of rain and then it stops just as suddenly..It's actually very welcoming when it's so hot!
Personally I stay up in the Hanalei area because it feels more like Kauai of years ago.
You can rest assured you can pack light because there is a Longs Drugs, and I think even a Costco
plus several grocery stores that arent really that more expensive than California. Also, find out
when the local farmers market is...lots of times they have a guy there with machete cracking coconuts
you drink right there...a GREAT experience for all! plus fresh veggies and fruits of the islands
I would also purchase the Entertainment book for Hawaii for coupons.
The book.... Kauai Revealed is an excellent resource.
My favorite cheap eats is Bubba Burgers...if I lived there ..it would be a place I would take (drag)you to..the burgers

are unbelievable...not to mention the attitude and ambience...they also have chicken and veggie burgers...google them for reviews!
You must try Lapperts ice cream..learn the history before you go. and ENJOY!

Another cool place to visit is the Guava Kai plantation..just google it...
everything you ever wanted to know about Guava....and the products to go with it...YUM!
I'm sure I'll think of more!
A hui hou!