I've never heard of hardening a HSS tool. I would question any one who claimed to have done it unless they have a pretty fancy oven and the knowledge to go with it. As far as I know, all high speed steels maintain their hardness right up to turning cherry red, you can't hurt the steel grinding with out damaging the wheel.
Plain Tx HSS is really hard to begin with, I use it for bits that get reground a lot because it grinds fast.
Mx has a hardness greater than Rockwell 60, and Cobalt is even harder. Cobalt tools are so hard they will actually chip when cutting hardened steels if you are not careful.
I use 1/2" to 3/4" cobalt tools for basic turning and facing because it wears for ever, but it takes a really long time to grind the cutter into shape. I like 1/4" or 5/16" M2 for threading because is grinds easier than cobalt but holds a better edge than T1. I also use M2 in flycutters because it handles interrupted cuts better than Cobalt.
It is hard to say if 4 hours is good or not with out knowing what type of HSS you are using, and what you are cutting. Tools get dull, this is just a fact of life. With HSS you just touch up the edge and carry on, I don't even remove the tool from the holder usually. I just take the entire tool + holder to the grinder and put a fresh edge on it. I also don't bother to touch the edge up with a stone 90% of the time.
If dressing the tool bothers you, then you will have accept the cost of replacing inserts, or try to find some Tantung bits. Tantung and similar steels are a major pain to grind because they rival carbide for hardness and can't be quenched.
