I love the new process version, and if I were in your position, I'd be sore tempted. But I'd have to advise against it. Using Beta software for anything that you'll feel bad about if everything falls apart is a bad idea.
Adobe have announced that they will convert Beta catalogues when LR4 comes out, but with the caveat that not all your adjustments may end up looking the same as they do in the Beta.
I would edit an image for print.
I would create a book for a non-professional purpose. (ie. a friend, a gift, a tribute)
I would edit an image for electronic output.
I would not process a client job, or any project that contained more than a handful of images.
I would realize anything I do, I might have to do over in the final release.
Just me and my comfort level with any beta software.
I would remember all the right points speaking against using the beta ... and would use it anyway
But your project seems to be both critical regading the time shedule and the mass of material, may be it's not a good idea. So at least you should do backups, backups, backups of you catalog.
BTW I think LR is not the perfect tool to do all the metadata work for such big numbers of files, may be you can split your work and first do the keywording etc. with another software and use LR4 in a second step.
While I like the 2012 development engine, I would not try to print from the Print Module in the current build. That is too cranky as I find the templates will not update on the fly or if they do, the performance is inconsistent. No issues of that type in LR3 and earlier. I cannot afford to waste time closing, re-opening, maybe re-booting etc for LR4 Beta to freeze. I would not do anything critical with a Beta version since it is intended for testers to find problems.
I know that the "official" answer is No - Don't. However I just returned (2/1/12) from a once in a lifetime trip to the Galapagos. I processed 1500 images (450 keepers) in LR4 Beta, including Export/Import as catalog from my laptop to my desktop, developing all images, putting up a web site and doing a book (which I have already received from Blurb). Absolutely no problems except a few forced restarts due to "cache" problems (but no lost work). Be aware, however, I am a retired guy, so worst case scenario is reloading my backup raw files from a backup device, and redoing a lot of it. Time expensive, but not catastrophic. I am totally satisfied.
But, I'd give it a go, knowing the concequences.
I put small shoots through, my racehorses in training and some more basic shoots, I've run a couple of magazine shoots through LR4.
Rob Cole and others have commented that there may be issues getting whites to white for deep etching, so a product shoot against a white background may be an issue.
I have a complex workflow for my mainstream clients, so that still goes through LR3. To stack, set white balances and export to TIFF, to enblend bracketted images and import the resultant TIFFS back to LR3, make the web gallery courtesy of theturninggate and then present that to my clients
Their chosen images then have the meta data written to them, and are exported as a catalog to a separate hard drive - to maintain the same folder structure so I can import them back into the catalog when LR4 final has imported my LR3 catalog. and these exported TIFFS are imported into LR4 and the meta data added, and all the edits done here.
As a professional I want to go back to the backup RAW images, but I do have frequent catalog backups, and I'm loving the LR4 improvements. I can get results far faster with the beta than with LR4, I am using the library, map, develop and web, I've never used the slideshow, I dont make books, and I've still not unpacked my fabulous Canon printer since I arrived in Cape Town 4 years ago.
However as most of the reviews of LR4 have indicated, there are very few changes to web and the library does not seem to do much more than it used to.
Yes I get crashes several times a day, but not once has LR4 told me it was having to "repair" the catalog, it just opens and I have to find the last catalog I was working on. The time taken for LR4 to apologise and contact Adobe is so little, that the time I save in editing 10 photos more than covers that crash 3 times.
However, I do back up my catalog several times a day.
Victoria, Hal, etc are correct, not a chance is the sensible option, as its a beta and with a new pipeline, process and prowess, and with bugs to iron out, and the beta catalog could crash and corrupt and you'd need to open a backup catalog, or it could even break the sidecars, or the metadata in the DNG files.
Re Hals comment that LR final may not read all the beta catalogs data correctly, that is an issue. But i've tweaked many images from LR3.x to LR4 already and 99% of my images go to clients and then can be thrown after they've received the final images. (Why should I store their photos? Its up to them to backup, I simply cant offer a storage service)
As for personal images, how many people go back to images 6 months to years later and fiddle a little? I do, so a tweak to make it LR4 final from LR4 beta is not an issue.
JUST SAY NO FOLKS, it may be nice, but its dangerous.