Once you pry the adapter off the Dorco Pace 4 cartridges so they can be used with the Headblade, is there any way that these blades differ?

They differ by sucking, four blades are trash. The three blade work much better.

How often do you replace the HB4 blades assuming you shave your head every other day?

I usually get 4-5 good shaves out of one cartridge. I have done more but I think you will be pushing it. You definitely feel it as you go. I most say that even if you can extend the life of the blade eventually the plastic spring base in which the blade mounts on will give out and no longer service it's purpose even with a brand new blade. That is actually my current problem. I have been shaving for almost 2 years now and have tried both the HB4 and HB6 systems. Definitely I prefer 4 blades over 6. Hope it helps...

Does this blade works for black men with bald heads?

I’m white with a bald head and I didn’t like it so I switched back to my Gillette Fusion blade. This blade doesn’t shave close enough and the shaving cream is greasy like Crisco in a can.

Did they change the blade type? Last two shipment are really bad. It came in different packaging than last 3 years and blades are not sharp at all.

Mine most recent order came in a bag, instead of the hard plastic container. I've used one of them and it is working fine. I didn't notice any difference between it and the previous blades. I'd go to HB and complain. I'm sure they will send you some new blades if you weren't satisfied.