DH4 is the code designator for the Bombardier Q400, a Dash 8 variant. Horizon operates the Q400, and that seat map can be accessed on Seatguru by looking under "Alaska/Horizon". The Q400 is actually an enjoyable aircraft, and even though it's a turboprop, the feel of the cabin is comparable to mainline aircraft. This isn't your rinky-dink prop job.

Your leg from Los Angeles to Medford is most likely an American codeshare with Horizon, and indeed Horizon flies between LAX and MFR with the Q400. Therefore when switching between American Eagle and Horizon, you'll have to make your way to the main AA terminal (assuming your Eagle flight gets sent to the remote terminal), then leave the sterile zone and trek it over to Terminal 3, where Alaska and Horizon is located. Some other FlyerTalkers that are seasoned with LAX can provide the quickest way to get there, but I believe it is just as good to walk rather than take the Inter-Terminal shuttles.

Enjoy!