An interesting move. I don't plat 3-4 often, so I haven't encountered it yet. Pro's don't play it either. Black looks a bit over-concentrated to me. I think there is no explicit punishment, but that the over concentration is the punishment.
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a was my first instinct, but seems a bit premature. Perhaps later, once my groups are more stable.
b and c calmly take a bit of a base. c might be a bit thin considering the closeness of blacks stones
d tries to take away his base. But a pincer seems annoying.
I'll probably go with b or c.