Ok, that code is indicating your ECU is reading the IMRC's are stuck open. It might just be a bad switch or might be actually stuck open. Really...the only way to find out what is going on is to pull the intake and physically look, not fun I know.
It should be ok to drive it until you get it looked at, but I would highly recommend getting this looked at sometime soon.
One thing you might could try is to run some seafoam through the intake and see if it might clear up any problems.
Here's a discussion on the same code on a ranger:
Dtc P2004 - Ford Truck Enthusiasts Forums
And another breakdown for you: