While socket head cap screws are harder to strip than Phillips head screws, it is possible if you try hard enough or are determined to use metric hex keys on imperial fasteners.
The new MSC catalog has the Sock-It-Out Screw Extractor for #6,#8,#10 & 1/4 Socket Head Cap Screws. I haven't tried them yet so I can't vouch for it, but the concept looks good. There is a metric set removes M3, M4, M5 and M6 but sadly for some reason they don't seem to make a set that removes #4 screws.
The distinguishing feature these extractors offer is that they do not require drilling.