Does Alistar's ult damage reduction stack with the new 4th dragon's damage reduction buff?

Does it stack against towers?

If it does stack, does it stack additively or multiplicatively?

If it is additively, then it means that can potentially become immune to tower damage with the 4th buff and his ult. This allows him to tank tower shots for his team during 7 seconds, allowing his team to dive with impunity.

Even if it stacks multiplicatively it is still a 79% damage reduction against turrets, which becomes 88% damage reduction with the 5th buff. This is assuming that both damage reduction modifiers stack, of course.