Hi Niklas,
To interface with a 20x4 LCD with Rasperry Pi using MATLAB, you will have to first download and install the Raspberry Pi Support Package. Once that's done you will have to use either I2C or SPI bus to read/write data to your LCD. You will have to refer to the datasheet of your LCD to find which interface types your LCD supports.
If I assume that you have to use I2C to inteface with the LCD then you can use the write function to write to the I2C address. There is a more detailed example available on that page, but briefly:
% Assuming mypi is your raspberry pi
% Get instance of the conenction to I2C device at address 0x55
i2cdisplay = i2cdev(mypi,'i2c-1','0x55')
% Now write to the display
write(i2cdisplay,[hex2dec('20') hex2dec('51')])
% Or to write to a register (register 3 here)
writeRegister(i2cdisplay,3,hex2dec('08'),'uint8')
