What are you using for a filter? You should have at least half of the below example to smooth out the signal. Also, it will never look too good on simulation. Might look better on actual hardware. Might be some issues with running it at 4 mhz. Is there a reason you don't want 20 mhz? Even AN655 's example showed 20 mhz.

Code:
;   Filter Diagram
;        
;               2.7k            2.7k    
;    RC2  ___/\  /\  /\______/\  /\  /\________ Analog Output
;              \/  \/    |     \/  \/     |  
;                        |                |
;                      ----- 0.1uF      ----- 0.1uF
;                      -----            -----
;                        |                |
;                       GND              GND
;
;