Predatory is the only important talent on the M4, the rest is up to interpretation. It's definitely nice to have healing when you have 0 stacks. It's better to wait to use the M700 until you have you have built some stacks (at 100 stacks you pretty much 1 shot everything).
The following missions it is better to have two M4s for more sustained damage:
I've used this build to solo all legendary missions. The following is how I rank difficulty from easiest to hardest:
Booster shot can be replaced with just about any skill. If you would like to try to force the enemy out of cover. Sticky Bomb, Seeker Mine, Shock Turret, Smart Cover, and Support Station are all perfectly acceptable. Pulse helps protect you from surprise pushes and flanks.
Talent wise Critical Save and On the Move are the only really important ones. Strike Back is a great alternative if you don't like one of the Talents.
Once the Blackout Event ends I will try to make youtube videos of each of the missions solo'd