4BIT DAC has negative voltage at op amp output
As you're using an inverting amplifier, this is kind of expected – in fact, not only kind of. This is your design intent - inverting the voltage.
If you don't want inverted voltage, don't use an inverting amplifier configuration for your opamp, but a non-inverting one (there's gazillions of things on the internet about opamp configurations, do your own research. Even Wikipedia has a page on opamp applications).