Twists are great! I do mine on damp or dry hair
When I do them on damp hair, I wash, condition, and wrap in a towel for a few minutes. Then I grab and twist, starting from the back. I make sure to moisturize. My fave right now is BB Oil Moisturizer.
There is no need for a product for hold as they will hold themselves! Type 4 hair has enough kink and curl to hold the twists without products. However, when I want extra hold for my twistout, I use aloe vera gel (lighter) or ORS Lock and Twist Gel (heavier).
When I do them dry, I start from a shake-n-go and just grab and twist as before, but this time I don't use product because I already have product from styling the shake-n-go. I do, however, spray water or some sort of watery leave-in on there to ease out tangles and prevent knotting.
In the front, be sure to twist how you want them to lay when finished. I put a short part in the front so it will have some shape.
For pics of twists, look in my album. I know that my April album specifically has dry twists.
http://public.fotki.com/Azure
Password is blue
Oh yeah, twists can last pretty much as long as you want them to. However, I would try for a week if trying them for the first time, and then work from there. I've heard of ladies who leave them in for two weeks or more, but they twist their hair a lot and know when it's at the point where it's about to loc!
Hope this lenghty post helped!