For the OpenRB-150, Serial3 is associated to D13 and D14, as you mentioned. And Pins D0 and D1 are associated to Serial4, while Pins D2 and D3 are associated to Serial5. So I am unclear about what you tried to do with your code.
If you are OK with reading English, please check my free Amazon Kindle book sample at this link
and read up more details in Section 2.4 for hardware and software set up for all 4 UART ports on the OpenRB-150.
May be you can use D4, D5, D6 or D7 instead of D1? I have used these digital pins as output before without any issue