Difference between H(A)+H(B) and H(A+B)
If + means concatenation, then $H(A+B+C+D)$ doesn't help make verification easier. You still need all leaves in order to verify any leaf, whereas the point of Merkle tree is to be able to verify individual leaves with only logarithmic cost.

If + means XOR or addition, then $H(A+B+C+D)$ is not collision-resistant so the construction fails to authenticate the leaves.