My endo doctor was actually stoked about my progress and my last reading, however he also warns me about the increased risk of going hypo as the A1c is coming down and I can see his point. As also mentioned by Lloyd a low A1c requires very strict control. An A1c between 4,8 and 5,2 is equivivalent to BG readings between 91 - 103 mg / dl and trying to stay in this range obviously increases your risk of going low unless you are able to enforce a very strict control.
I do admire those in this forum who are able to, and want to do this - I am not one of them - but if I can get myself down around 6% I believe I have minimised my risk of complications and I can live my life (diet wise) in a way that is managable for me without being too concerned about going hypo.
If you decide to aim at an A1c in the 4's please do it slowly as also mentioned by others. All the best!