Why do i only read a value from 1 of 4 multiplexed analog signals?

Using Arduino LEDs and Multiplexing
2

Is only one phototransistor illuminated at any time?
You could effect an R/2R DAC with some additional components

Also, if you could tell me how to print all 4 values on one line, that would be helpful. i already tried Serial.println(val1 + ", " + val2 + ... and so on);

Serial.print(val1,DEC);
Serial.print(" "); // gap
Serial.print(val2,DEC);
Serial.print(" "); // gap
Serial.print(val3,DEC);
Serial.print(" "); // gap
Serial.println(val4,DEC);

BTW, your "PTs" aren't multiplexed, they're just in parallel.