I was playing Gorod Krovi with a friend, I got the L4 siege from the mystery. The issue is that for what ever reason, because i wasn't the host the weapon decided not to function properly. The rockets are suppose arc upwards from the weapon then all out launch. What happened was instead of the rockets going up, they went down and exploded instantly on the ground. if you aren't careful and adjust to this, this WILL get you killed so be wary when using the weapon in co-op.
 The second is there is currently a bug that prevents the player from being able to switch weapons. I think monkey bombs may cause it, however I can't 100% confirm it. The good news is the player can easily fix it buy buying a new weapon, perk, by hitting shift tab, alt tab, or by pausing the game.

the L4 siege works different in zombies compared to MP i think, what you described is how the weapon is meant to work.

the L4 siege works different in zombies compared to MP i think, what you described is how the weapon is meant to work.

I thouvht that at first too, until i loaded a solo game and had the rockets do the same thing they do in mp.