I think gmaxwell is right, there is not enough evidence in those 4 blocks to suggest that there has been a breakthrough in mining ASICs.

On the other hand I guess valiron is also right, he knows (and probably can't disclose) that some mining companies may be testing new tricks to improve their ASIC hashing power. Maybe he hacked into one company computer or he was told by a friend who works in one of them and he promised not to say anything.

Mining companies want to maximize their profits and they are not obligated to disclose their engineering achievements. Those are trade secrets and nobody has ever complained about ASIC Mining companies having close designs. I don't even think that the Bitcoin community can even enforce ASIC companies to open their designs, because they are already using other companies IPs that they can't disclose, and because they can always go anonymous to avoid disclosing anything.

There is plenty of technical information put together in this long thread (given by gmaxwell explicitly, and DannyHamilton's analysis on which parts of the block header can be used as nonce, and my very old posts about modifications to the Bitcoin header) to ease you discover one of such tricks. Take some time to think about it, take aside all posts with personal insults, and you'll probably find the solution right in front of your eyes.

I'm not that clever so there may be more tricks to discover.

However, a trick can only give you a certain speedup, say 20%, based on a reorganization of the SHA256D operations, or the pre-computation of some operations that change less often. Other changes (such as reducing the fabrication node) can give you much higher speedups. So this isn't alarming.

A completely different thing is to find a way to invert SHA256D, which I'm absolutely sure nobody will ever be able to do without some revolutionary quantum computer that does not exists even in theory.

The only attack I was thinking of when I wrote the Bitcoin header post, was all mining companies adopting tricks that give them some little advantage, but at the same time they degrade the performance of the network as a by-product. One of such attacks is cited when I posted about using approximate adders, and the danger that a monoculture of approximate ASICs can get stuck in a header that always generates a faulty addition.
 
If such problem ever arises, the community will probably find the way out by doing the right hard fork to prevent it.

IMHO the cryptographic security of SHA256D function of Bitcoin will never be seriously compromised.
However if there were a single mining company manufacturing ASICs being 200% faster than the competition, that would clearly hurt Bitcoin in a practical econo-socio-political way. The good news is that the accumulation of tricks probably will never reach such an improvement.

Best regards,
 Sergio.