There are 3 tiers to Back 4 Blood's pre-purchase. Here is a breakdown of what is included with each.
(Note: these may be subject to change, pray for our sakes that it does)
Back 4 Blood Standard
Contents:
Back 4 Blood Game
Fort Hope Elite Weapon Skin Pack

No season pass/upcoming paid campaigns, and no extra skins. So in other words, nothing.

Back 4 Blood Deluxe
Contents
Back 4 Blood Game
4 days early access.
Annual Pass (upcoming DLC campaigns and playable characters)

Not terrible, but we can go deeper.

Back 4 Blood Special Edition
Contents
Base Game
4 days Early Access to Back 4 Blood
Annual Pass: Three upcoming downloadable content drops with New Story, Playable Characters, Special Mutated Ridden, and more
4 Character Battle Hardened Skin Pack
Additional digital in-game items: Rare Banner, Emblem, Spray, Title

For all you folks who absolutely can not stand the idea of being in possession of currency, have I got the deal for you!

Now that you've discovered your deep hatred for money, it's time to rid yourself of that which has brought you so much pain. That's right people, whip out that 3 inch satisfier .

If you truly harbor an immense hatred for money, pick up the Special Edition. I mean, come on, it's SPECIAL EDITION! That means it's better! Just look at Star Wars!

Now, if you're slightly on the fence about your stance on money, like I dunno, maybe you're not fully sure it what it's done to you is worth all the hate, then buy the Deluxe edition instead.

You're terrible.

Now that you've eliminated the root of your hate problems, you win! Turtle Rock Studios can now enjoy your money!

And remember kids, running with scissors will give you a passive speed buff, try it some time.