Once you have handed four Dvalin in to the gunsmith the next step is a mission on Earth called "First Firewall". So keep checking for it each day. HOWEVER note it is very rare. So far in all the time since TTK started it has only appeared twice. It must be getting on for a month since it last appeared so it is probably due quite soon. There may be no pattern to it but the last time it appeared was a Satruday so it might be a Saturday when it appears. To be honest, at this stage, I don't know why Bungie are time gating this so sparingly. Everyone who really wanted one probably already got it in the first or second go so I don't see why they wouldn't simply make the mission available all the time for those who have the four Dvalin or at least maybe once a week?