chris3145:
Awesome, thanks! If I understand correctly, that means that computer connected to RX can use RS232 as long as the screen uses TTL?
No I believe that is incorrect. The Arduino speaks at TTL levels so you will need a level shifter if the computer output is RS232.
One more thing. If you use the hardware serial in this manner, it is going to limit your ability to debug print to the serial monitor as you build. I am using a 4D display on a project and I am using softwareserial to talk to it so i can still use the serial monitor. In the finished project, the hardware serial will connect to an Anaren 915Mhz radio module.
Are you using the display in SGC or GFX mode? If you are using SGC mode, you will probably want to receive the ACK reply when you send commands. I didn't do this for a long time and ran into a bunch of issues. Once I started looking for that ACK, I found and corrected all of my problems. If you are using GFX mode, this is a moot point. I personally chose SGC mode so I could keep all my code in one place. If you are going to go with the SGC mode, there is a great library already created for these displays. I didn't use the whole library because of ram considerations and I didn't need access to every function included in the library.