No, hits on the defender also refer to a 4th hit.
Also, while doing a little testing, I set up a situation where one armor unit assualted a lone AT unit. Although the result called for 1 hit on the AT (i.e. D1 in the CRT), there was no hit in the game. I'm not %100 sure, but it looked like either a D2 or D3 result was needed to score a hit. This apparent anomaly only happened when assualting a LONE AT unit. I just thought that maybe the way hit priorities are handled might be the reason. If you get a chance, could you give me a "sanity" check on this?
BTW, I'm really enjoying this game. For such a small footprint (approx. 200mb), there's an awfull lot to it.