This information applies to the original FTM-400 radio. Radios purchased after about June 2016 already have the firmware upgrade needed to use the SCU-20 cable for programming. If you purchased your radio after 2016, you will not need the information of this article. 
Programming the Yaesu FTM-400 radio is done with the RT Systems ADMS-M400 Programmer. You can program any FTM-400 radio using a micro SD Card. For radios with firmware version 2.0 or higher, you can use the SCU-20 cable that comes with the radio. The RT Systems software will work with the Yaesu SCU-20 cable.

Only your radio can tell you what firmware version it is. To check:
Step 1: Press and hold for 1 second the [DISP/SETUP] key on the face of the radio to access the menu FTM-400 Front

Step 2: From the menu, select RESET/CLONE.

Don't worry, you won't reset your radio.
FTM-400 Menu

You will find the radio version number at the top of the screen that appears. "Main: 2.0" is what you are looking for. FTM-400 Backup

Click "Back" twice to exit.

If your radio is less than version 2.0, Yaesu has released a firmware upgrade to address the earlier radios.

This is Yaesu's upgrade. They are available at 714-827-7600 for questions. Check at Yaesu's web site for the latest firmware upgrade for that radio.

If you were using the RT Systems ADMS-M400 programmer prior to doing the firmware upgrade on the radio, be sure to complete Communications| Get data from radio into a New file (File | New) once the upgrade is completed to tell the programmer that the radio has been upgraded. Once this process finishes, you can send your original file to the radio (Communications | Send data to radio) without having to re-enter a single detail.