use 2 inch eternabond instead of 4 or 6 inch????

If you're putting it down over chaulking sometime 2 inch may be just a little close. But I would rather have 1-2 inches of tape on each side of the seam as a good barrier to water instead of maybe 1/2-1 inch. It don't cost that much different to get better protection..

I went with 4" - probably overkill for side seams but needed that width for overhead seam on Class C. I'd rather have more than less when it comes to avoiding water intrusion.

It totally depends upon your roof and how much you need to overlap.

On the basis that I only ever wanted to do it once, spending the $100 on 2 rolls of 4" was worthwhile compared to the $60 for the 2". It overlaps more than it probably needs to, but more is better than too little.

If all you are covering is a hairline join between seams then I am sure that 2" is sufficient - in my case I had to also cover a raised transition strip between the panels and so I used 4"

2 inch works for me to cover seams, hardware and transitions.

We use the 2" wide tape too, works perfectly.

2" wide worked well for me.

The 4" isn't that much more expensive and it's a whole lot easier to get a good overlap past the screw holes and onto a good clean section of roof.