What is the maximum number of guards that can be assigned to the vault door room fully upgraded?

While trying to decide which defense strategy is best to guard my vault against raiders, I came across some conflicting information about the room occupancy.

Some say that the only benefit we get from upgrading the vault door is that it gives us more time before raiders enter the vault (so I assume it will still hold only 2 guards), while James says in this great answer about room layout strategy) that you can place four guards at the gate.

Do NOT use the gate room for defense. You can only put four people in there.. its just not as effective as a full sized room can be.

So, what is the maximum number of guards that can be assigned to a fully upgraded vault door room? 2 or 4?

If it's 4, I'll fork over some caps to upgrade the room and have my guards there to minimize raiders time inside the vault, while also having a 6 people room next door armed to the teeth.

If it's 2, I'll save some caps and won't bother upgrading the room. I'll leave the guards in the room next door.

Solution 1:

While it looks like it should be able to hold four dwellers, I can only assign two dwellers to my fully upgraded vault door.