A drilled pier with a projection of 4' above grade is a rather small projection, just make sure your formwork or sonotube can withstand the lateral pressure of wet concrete.
How many drilled piers do you have, 4? 400? If you only have a few, I doubt that you would see any significant difference in cost regardless of which method you choose.
Also, if you extended the drilled pier up 4' above grade, does that eliminate a trade (i.e. steel fabricator and erector)? If not, and you will be erecting steel anyway, again I say the difference in cost is negligible.