Does anyone have experience trying to get a Grab taxi around 4am. I need to go from Khlong Song Rangsit to Don Muang Airport for an early flight and I am wondering if I can depend on them or try to make other arrangements. 

Pre-book it in the app. If you can't pre-book because it's full, allow extra time because they may not have drivers available. But I've never had trouble getting a car at that time. It starts to get difficult after about 6 am, but it probably depends a lot on location.

I love an early morning grab.

You shouldn’t have any real issues at that time.  But always on the safe side give yourself some extra time. 
One thing you should be aware of regarding pre booking.  Which I have tried at times.  Just because a driver accepts a pre booking,  they can still cancel.  Even when they are on the way to your location.  This happened to me a couple of times while living in Bangkok.  

I take early morning flights pretty often and I've never had any problem using Grab for a ride to the airport at 4am.

10 hours ago, swm59nj said:

One thing you should be aware of regarding pre booking.  Which I have tried at times.  Just because a driver accepts a pre booking,  they can still cancel.  Even when they are on the way to your location.  This happened to me a couple of times while living in Bangkok.  

Another thing that you should be aware of is if that happens you just use the Grab app to report the driver who will be reprimanded and disadvantaged on the app.  Grab advises you of the outcome of the complaint.