Looking for tips on learning more about exotic spheres specifically in the 4th dimension. I've been going down the wikipedia hole looking at perelman, poincare, milnor and trying to grasp what they found. I have some pages telling me it is unknown how many exotic spheres exist in dimension 4, then others saying that there are large and small exotic R4's. I am going through published papers but there are a lot, any help would be appreciated.

>tl;dr
>looking for helpful topological papers on exotic spheres in R4