You would also need to be mindful for reciprocity. States may not grant you a license, or give you trouble, if you get a license too early.
Probably best to wait 4 years.
Do you have any experience/proof of this or is it just your opinion?
Certainly, you should contact your state licensing board before taking the exam but I disagree wholeheartedly with the previous poster: take the exam as soon as you can even if you need more work experience before your state can license you. Taking it straight out of college or grad school can be very beneficial for the following reasons:
1) The material is still fresh in your head. While hands-on experience will obviously help you with certain parts of the exam you might never encounter certain topics on the exam ever again after school (at least that has been the case for me in enviro field until now).
2) If you still have not landed a job after school use the time to study and prepare for the exam.
3) You don't know where you will be in four years - you might be married with 2 kids and with a third on the way. Studying will have to take a back seat to changing diapers and 3am feedings.
However, I can't speak for myself, because I only just took the enviro exam in October 2012 and although I took it right out of grad school I also had 3 years of experience. My 3 years experience did not help me much on the exam but almost everything I learned in grad school was on the exam (plus other material unfortunately that I never learned in school or on the job).