How to make a sawtooth waveform look less like a staircase (DAC)?
A stairstep waveform means that your DAC lacks an appropriate reconstruction filter:

https://en.m.wikipedia.org/wiki/Reconstruction_filter

In this case, an RC filter set to less than half the sampling frequency (ideally much less for a low order filter) will work.

Adding one will give you a smooth waveform, even with only 4 bits (but your SNR will be limited to 24 dB by quantization noise unless you add more bits or oversample).