I have both the 3.0 and 4.0 and they work in all of my DeWalt tools (drill, impact, saws, etc.)
Yes
Yes, the same charger can be used for all the 20-Volt batteries.
any 20V charger will work, but it's worth it to get the fast charger because it does charge these big batteries much faster.
yes
I use mine on a leaf blower and it doesn't feel any different than the 3ah that I had been using. The weight difference is nominal even holding one in each hand at the same time.
Yes, they will work in the regular 20V DeWalt tools and charge with the regular 20V DeWalt charger. I have been very happy with mine.
with the adapter all the 20V system batteries will work
Just under a pound.
My brand new fully changed 20V Dewalt battery lasted maybe 5 minutes if I was lucky. My 10 year old Craftsman 19.2v NiCad battery lasted 3 times and long. Which is way shorted than it did when it was brand new. Which is why I just bought the DeWalt. I'm very disappointed. I thought DeWalt was supposed to be quality stuff.