I think the reason 4 hours is listed as the time to treat water for crypto and other hard to kill nasties (yeah, I know, a really technical term there) is that 4 hours is about the maximum time oxidants will remain about in the water at the standard dosage levels. So all you are really being told to do is treat the water for the maximum dwell time at the standard dosage.
On the topic of filtering vs treating…
I stopped filtering water for personal use ages ago because of the obvious weight savings of not carrying a filter in lieu of chemical treatments. I haven't come across a situation yet where the chemical treatments have fallen short for me.
That said, when I camp with the Boy Scouts I take the "belt and suspenders" approach. I have the boys filter the water… it's good for them and gets them out of my hair for a while. Then when they get back I treat it with Aqua Mira. I do NOT want to return a boy home from a backpacking trip having to explain to his mom why he has to be treated for giardia from drinking flowage water.
When I travel in a group I have frequently run into situations where people have asked for water or I have offered to share water. I always make sure to inform people how the water was treated and answer any questions they may have. I think it's important to recognize that so much of this topic is a matter of personal preference and a lot of factors are at play. I don't just mean pros vs cons of each treatment method but rather the sum total of the preferences and personal experiences (emperical history) of any given backpacker.
I will say, and this may draw some heat from the forum, that I think these hikes where people opt to NOT treat water and risk getting sick in order to save time on fast hikes is more than a little nuts. "If I contract giardia it will take a week to set in and I hope to be off the trail by then." seems a bit crazy to me. Not my cup of meat, I guess. ;)