Vce saturation voltage confusion
Why is the real saturation voltage way higher than stated in the
  datasheet?


Because you have configured your transistors as a darlington pair and no matter how hard Q4 is turned on it can only realistically shorts Q3's collector to Q3's base and, given that you need 0.7 volts on Q3's base to turn on Q3, you are left with Q3's collector at about 0.7 volts.

If Q4's collector were connected to 5 volts via an appropriate resistor (about 390 ohm) it would work closer to how you expect it to because you can source >10 mA from the 5 volt supply into Q3's base.