the full licence of version 4 lightroom is so much cheaper then the version 3 when I start using it.
you can get lightroom from Adobe site, just download the trial version to try it out, it is fully functional as licence copy but only good for 14 days.
if you like it, just purchase the licence key from Adobe site, you will need a Credit Card to make purchase, sadly they don't accept Paypal. (IIRC, Paypal can't use for software purchasing)
if you just purchase the software, what you paid for is the current version, all the patch files are free under the same version you have, you will be notify for any update if you are a registered user.
if lightroom release a new version, you still able to use the older version you have, or take up the offer of upgrading it to new version, it cost of upgrading is about half price of what you have paid before.
for some people still prefer to purchase the box version, but IMO it is not necessary, you will be paying extra for the packaging box, and there is no instruction manual in it, just a CD and licence key, and after you install lightroom in your computer with the CD and register with Adobbe, probably you will be prompted to download the software with the latest patch immediately. isn't it paying extra and doing extra work unnecessary?
I got my full boxed retail version at Sim Lim Square for $149 nett. Just ask the software sellers. I think I bought mine at the software shop at Level 4, near Best Price and Fuwell. I remember LR4 is selling at around the same price at several software shops there.