What's happening to my DAC signal here?
Why is my signal looking at that as its under sampling? 


It doesn't look undersampled, rather you have no reconstruction filter so you get a stair-step waveform.  Add the filter and you'll get the expected reconstructed signal.


  why isn't the 4.3 kHz signal not looking like the 50Hz one?


Because you changed the time base on your oscilloscope.  A combination of filtering your scope at lower frequencies and/or undersampling gives you something that looks more like the output of a reconstruction filter.  Zoom back in and you should see that it is still a stair-step.