It would depend, I suppose, on what features you borrowed from L4D, and what features you kept of Halo's.
Speaking only from a campaign perspective, what makes Halo, Halo?
Semi-nonlinear, open environments. The Halo sandbox combat and physics. Vehicles. Recharging shields. Two weapon maximum.
What makes L4D, L4D?
Mostly linear levels. Focus on cooperation with medkits, pills, and reviving. Endless hordes of enemies. Interactive environments with scripted set-pieces. And last but not least, an innovative hybrid versus mode that combine co-op with player controlled enemies.
Do I think they could be combined to some extent? Maybe.
At the very least, a 4 player co-op Halo campaign with 4 players also controlling covenant enemies would be rather interesting.