  Motivated by a recent paper of Gabai on the Whitehead contractible
3-manifold, we investigate contractible manifolds $M^n$ which decompose or
split as $M^n = A \cup_C B$ where $A,B,C \approx \mathbb{R}^n$ or $A,B,C
\approx \mathbb{B}^n$. Of particular interest to us is the case $n=4.$ Our main
results exhibit large collections of $4$-manifolds that split in this manner.
