Brother GTX-4 Instruction Manual page 166

Check the print image displayed in the preview window, and press
(4)
<IMPORTANT>

Never touch the USB flash memory when the print button is blinking. Otherwise, any already-saved data may be corrupted.
<TIPS>

Depending on data size, it may take several minutes before the loading process is completed.
 If any files of USB flash memory cannot be read out, please refer to "9-1. Troubleshooting instructions >>P.236" for details.

If a folder contains more than one print data, you can use
The print data loading will be completed when the print button stops blinking and remains lit, while the electric chime sounds.
.
buttons to switch the preview display screen.
166
Basic printing procedures