BCD Switch Resistor Network for Proportional Voltage Output
You could use use buffers after the switch as your solution #1 with an R-2R network. A 74HC4050 or 74HC4049 depending on whether the switch is inverting or not. 

Or buy resistors that are in the 1:2:4:8 ratio- there are only four parts or so- there's no need for a network. Eg. 10K/20K/40K (two 20K in series)/80.6K. You don't (necessarily) need a constant current source- for example you could use the switch between a negative voltage reference and the input of an inverting amplifier.