To be honest, I would really have to disagree with the 2 mil into the 4 mil.
So, if you are doing that, that already shows me that you are a serious collector and you know how to handle your books. So, why do you really need that extra thickness if you know how to handle your comics?
The 2 mil isn't sized correctly and it has extra mylar at the top as does the 1 mil. Placing a board into that makes the top (where there is no board) bubble and it can curve your books. By placing them into a 4 mil, you cause them to be able to curve more over time.
The 1 mil is thinner and easier to crease and make that bubble go away with pressure. If you pull the mylite over the board, that extra space becomes very unnoticable and will not curve your book near as much.
In the end, I do not believe that by going with the 2 mil offers you really any more protection than you really need. It will cost you less to go with the 1 mil and will benefit your books in the long run.
As a seller of these, the capitalist thing to do would tell you to go with the 2 mils because they cost more and there is more profit to be had, but I honestly can't do that because I can see the damage that can happen from it.