My 3.5 yr old declawed house cat suddenly took off out the door and we haven't seen her since. Is this normal for a cat to all of a sudden want to be outside in 40 degree temperature, when all she has known was the great indoors? I am extremely worried and scared for her. She has never been outside since we've found her at 5-weeks of age. What should I do? We have been scarrowing the neighborhood morning and night and no sign of her.

Is your cat spayed or not? If, not, it could certainly be 'nature' calling if she's taken off.
Guessing she is spayed, let's move on.
I once moved into a new apartment w/ 2 cats. As soon as they arrived (indoors) they completely disappeared and I could not find them for 24 hours. I kept looking in every place I hadn't looked and eventually found them deep inside a roll of new carpeting. They stayed in there for 3 solid days--never ate, drank, or pooped or peed (thank goodness). I guess terror shut down all body function? Cats terrified in a new surrounding--which outdoors to an indoor cat can be--look for a place to hide. Look for cat-sized places to hide. I kept a bowl of tempting canned food (they normally ate dry) a few yards from the carpet roll and eventually first one, then the other, sneaked out to eat. They spent the first day our walking the perimeter of the walls, not wanting to venture across wide open spaces. The concept is stay safe and hidden from any larger predators, I think-'-hawks diving from above' is probably hard wired into a kitty brain whether they've been outdoors to fight off larger predators or not.
Similarly, we moved to a new home once with another cat who was exclusively outdoors. Moved from 5 acres to a small fenced city yard. Let the cat out and she immediately disappeared. We figured she'd hopped the fence and was long gone. But days later we discovered her beneath a concrete porch--a space in the dirt underneath just small enough for her to crawl into. We found her because our dogs kept sniffing there. Again, we put out food and eventually she came out and acclimated herself to the new yard, in fact never once hoped the fence and stayed in that yard for 3 years.
So I'd A. look for nearby cat-sized spaces to hide in (use a flashlight to see her eyes reflect) and B. put out delicious canned food and see if it disappears. One morning you may just find her on your porch begging to come back in.