Solution
1. Nd6! cxd6 2. Nxc6+ Kd7 3. Nxb8+ Kc7 4. Qxe6
The point of 1. Nd6 is that it threatens the discovered attack Nxc6+, followed by Nxb8+ and/or Qxe6.
Or
1. Nd6 Qxd6 2. Nf7+ followed by Nxd6
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1. Nd6! cxd6 2. Nxc6+ Kd7 3. Nxb8+ Kc7 4. Qxe6
The point of 1. Nd6 is that it threatens the discovered attack Nxc6+, followed by Nxb8+ and/or Qxe6.
Or
1. Nd6 Qxd6 2. Nf7+ followed by Nxd6