I have the same problem. I've turned in 3 but the last one is not dropping for me since TTK launch.
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Same here. I have put in at least 40 hours of pve into ttk and cannot get the last fusion rifle. It's a tad frustrating. No quest dropped for me, so I know I did not have all 4. Best of luck to all. If it's a glitch, please fix bungie.