Until recently, A4 was reserved for future use in certain direct injection engines.
Its just an improvement, or change upon previous specs that the manufacturer - in this case Hyundai - feel is best suited to their engine. Where you have to be careful is :
A) Hyundai, originally cited two specs, A3 and C3 for the same engine; the C3 low ash was brought in due to issues with exhaust valves, at the same time their literature, manual and other info was still stating A3 - so in the event of a warranty claim you'd be covered both ways. Albeit, the change to C3 low ash was introduced when these issues with the exhaust valves were highlighted.
However...
What your dealer uses is anyone's guess. During a long debate on the forum over oil, a few dealers were asked what they used. It ranged from A3 semi synth, to A3 fully synth and everything in-between, with only a few stating they used C3 low ash as recommended. Thus, if warranties were going to made void in the event of engine failure (including an analysis of the engines oil), then many dealer serviced cars would have not been covered.
The moral : Use whatever Hyundai recommend, in this case A4, and make sure your dealer is using it - even though you have to accept that is what they are using !