Oh bummer.
You really need to talk to your client, and ask to be able to communicate directly with whoever will be doing the printing. Stress the importance of this, from a quality point of view. He will be receptive, I'm sure.
Once you have the printer's details, contact them and ask them what resolution they need the file to be, and also if they need any bleed (it's likely they'll say the exact dimensions of 1200x4800 will be fine, but let's not leave anything to chance - it's possible they might need a few extra millimetres to allow for trimming).