Oh God, the wait is gonna be torture! I'm not getting married until late 2015 and people think I'm crazy, so I dont even mention that I'm actually already 6 years into planning! lol
There is certainly no harm in having a look around and getting a feel for what you like, It turned out that where I thought I wanted to have my wedding wasn't very welcoming, so glad I have plenty of time to look at other options!
My only advice would be that practically no venue will be able to take bookings so far in advance, and nor should you want to put any money down so early, alot can happen in those 4 years, and many people pospone things for all kinds of reasons, 2 years/18 months is more than enough notice. MOst venues and suppliers will look at you like you are crazy, they did to me, and alot still do it, so if you can get away with not telling them a date yet, dont, I found by not disclosing the year, they took me more seriously, after all, I wanted a rough idea of they charge for things, even if this price might have doubled when it comes to book!
Its never too early looking for ideas though, and definately never too early to start slotting some money away for it either!!
On a more personal note, I love the fact there is someone out there closer to my dates!