You can jump right to the one you want
I'm doing the JLPT 3 in New York this year, and I've never done 4.
In terms of preparing for it, one of the best things I can recommend is to download old JTPT tests from previous years. While the questions won't be the same, it'll give you a really good indicator on how you measure up to that level
I tried to search but I had a hard time finding them @_@ but they are out there, I promise!
I did stumble across this though:
http://www.manythings.org/japanese/v...pt3-adj-i.html
Also, if you have a facebook, start using the application called Kanji Box. You select the level of the JLPT you're going for, and it'll quiz you on tons of kanji/vocab you need to know. It's great!