Here is how you can get SLI working in Fallout 4:

1. Download Nvidia Inspector from 
http://www.guru3d.com/files-get/nvidia-inspector-download,4.html


2. Click the Wrench Icon to bring up the Profile Settings window.


3. Under Profiles, click the dropdown and select 'Fallout 4'


4. Under Compatibility, change the setting for "SLI compatibility bits (DX1x)" to 
"0x080200F5 (Max Payne 3)" by clicking the dropdown next to it.


5. In Nvidia Inspector, scroll down to the SLI section.


6. Here is what you need to change under SLI:
   
Number of GPUs to use on the SLI rendering mode = SLI_GPU_COUNT_TWO 
(You can select FOUR if you have four graphics cards in SLI mode. Select accordingly.)

NVIDIA predefined number of GPUs to use on SLI rendering mode = SLI_PREDEFINED_GPU_COUNT_AUTOSELECT

NVIDIA predefined SLI mode on DirectX 10 = SLI_PREDEFINED_MODE_DX10_FORCE_AFR2


8. Run the game and enjoy the increase number of frames per second!
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Here are how my settings look: http://i.imgur.com/832vCOd.png
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