You don’t need to show any identification using Disney transport.
I’d be a little weary of having only booked for 4, and actually being a group of 5. Disney will have fire and safety issues with 5 in a room for 4, the room occupancy is set by the fire marshalls. The CM at check in might just happen to notice there are 5 of you checking in too. Not really sure about how this will turn out! In a way, you are ‘lying’ to Disney and they do frown on such matters.
Did you book online or over the phone? Usually the online booking systems will automatically charge you for the additional room but if you make the bookig over the phone, I’m pretty sure given the age of you’re children you’d be able to get just the one room and possible on pay fees on a trundle bed?
Your resort keys are your park passes so if you only have 4 guests in your room, you’ll only have 4 park tickets. They no longer give out EMH wristbands (at least for morning EMH). Without that 5th resort key, someone isn’t getting to the parks for the duration of your stay. Unless you have only booked the accom with Disney and tickets from another source?
I’m sure someone will come along with more information. In the mean time, I would consider ringing Disney and getting the guest # rectified.