Does Lotus Trap double Monstrous Harpoon's damage if both are played during an attack?

I have Lotus Trap hidden on a battlefield and a Watcher attacks it. If I play Monstrous Harpoon from hand and then react with Lotus Trap, does it deal -4 or -8? Does Monstrous Harpoon's damage trigger when the spell is put on the chain or when it resolves?

Does Lotus Trap double Monstrous Harpoon's damage if both are played during an attack?