Agreed with the above (didn't we just have a discussion on this?), but will add one thing:

The Diamond breakfast benefit applies to up to 4 registered guests staying in the same room as the diamond member.

Many properties will incur a separate additional charge for anything more than 2 guests staying in a room. It is completely within reason for the hotel to ask you to pay an additional guest charge for guests 3 and 4 if you make it known you expect a breakfast benefit for them.

That said, most properties are generous and will grant 4 vouchers for 4 guests in 2 rooms on the same reservation as long as the Diamond member is one of them. Just recognize this for what it is (generosity not required by the program) and be prepared for a property to stick to the terms.