In order to install that font on your phone you will need to root it. Since there isn't an official website with this method I suggest you search on Google for "root T-Mobile My Touch 4G" and follow the third link.
After you complete the rooting process, download Khmer font and place it on the internal/external storage of your card. After that you will need to use phone's file manager to browse to /system/fonts. Copy the font in that location and reboot the phone. Once it starts you will need to go to Settings > Display > Font and choose it from that menu and you're done.
Note: Rooting your phone might void its warranty. Please be careful!