There's decent data for K1, and multiple forms of K2, including mk-7 and mk-4. The type of data supporting each is currently different, a testament to the infancy of the state of knowledge on this topic.
There's a randomized placebo controlled trial in humans showing, as a secondary endpoint, drastic reductions of cancer among otherwise D3 replete women (mean serum level 30ng/ml) who took 5mg of k1 (a big dose). There's also generally good safety date for k1.
K2 in the mk-7 form has good epidemiological data, though I suspect some of the benefits that K2 shows in these studies is really due to the benefits of eating dairy fat (but the researchers can't accept that so they attribute it to K2). Same phenomenon for CLA.
Mk-4 has decent animal data behind it and some short-term trials in people (but only at massive doses that, in my opinion, would be quite unwise to take for purposes of disease prevention).
Yes, it might even be the most powerful form. MK-7 is found in fermented foods and that is basically the one your bacteria make in intestines.
MK-7 has a profound ability at preventing osteoporosis, and even at reversing it and calcification. It is the main stuff in natto and also one of dominant forms in human serum.
Ideally you should aim for mix of all K vitamins. Fermented foods contain other MK forms (8,3, 9 etc..) with possible different roles in the body.
It looks like its very safe. This is interesting
Therefore, under the conditions of this experiment, the median lethal dose (LD50) of MK-7 after a single oral administration in mice was determined to be greater than the limit dose level of 2000???mg/kg body weight. The no observed adverse effect level (NOAEL) of MK-7, when administered orally to rats for 90 days, was considered to be equal to 10???mg/kg body weight/day, the highest dose tested, based on lack of toxicity during the 90-day study period
This striking lack of toxicities with orthomolecular compunds relative to pharmaceuticlas is the reason why Pauling created new medicine. Ofcourse, rat observations do not necessarily translate to humans, but since you can't test LD-50 on humans its reasonable alternative.
See this table for dietary reference.
My recollection is that the MK-4 K2 variant is fast acting and cleared from the system quickly, and MK-7 lasts longer in the blood. So it's definitely worth taking it if you've got it, although complex's of the K vitamins are better. The various K vitamins help out with osteoporosis, arterial calcification, cancers, etc. and the K2-MK7 variant is not really in our diets (unless you eat Natto a lot, not common in the US diet)