Hi, I'm Pete. Welcome to JustAnswer! I'm reviewing your question now, and will post back with your reply ASAP.
The iPhone itself only has one speaker, however by default it should play out of both speakers when using external speakers - there is no setting involved on the phone, so it sounds like you might have an issue with the hardware.
Have you tried restarting the iPhone?
Have you tested the speaker with another phone?
Have you checked the speaker balance settings on the DEH-P4000UB when the phone is connected?
And all phones are affected in the same way, yes?