Can I set the slider on my universal adapter between 4.5V and 6V to get 5V?
If the slider is a switch, then you won't be able to set the output to an intermediate value (5.0V). The only way that you would be able to get a 5.0V output is if the slider is a linear potentiometer, although I find that unlikely. 

It's more likely to be a switch because switches are cheaper and don't require calibration for specific positions (4.5V, 6.0V, etc...). It may still work at the 4.5V setting, depending on the electronics since 4.5V is only -10% from the specified voltage.