A 750Ti can run Fallout 4 @60fps , but your GPU is holding back your CPU. Your CPU is sending out information faster than your GPU can handle/display on your screen.
Fallout 4 has v-sync forced on by default. it caping @ 40 leads me to believe your monitors refresh rate is capped at 40hz. You can disable v-sync in FO4 though: https://steamcommunity.com/sharedfiles/filedetails/?id=551069501&tscn=1447077828
You'll probably have to cap your FPS to 60 as well, because the games speed is literally tied to the FPS, so anything above 80 FPS makes the game run uncontrollably fast.