Thus far we haven't made any effort to provide upgrade scripts for the changes we've made during development. The datamodel is explicitly unstable until we hit "beta". We're nearing that point but aren't quite there yet.
People who've built OpenACS 4 sites while its been under development have understood that until it stabilizes, they're at risk of being somewhat orphaned or having to figure out their own datamodel migration path. Particularly using Postgres - which is one reason why the few serious sites out there based on our pre-release code are Oracle, not Postgres, based (the Oracle data model's been relatively stable).
In the future we do plan to provide upgrade scripts. The package model is a big help here, if someone vastly rewrites (say) the bboard package and datamodel (as I'm tempted to do everytime I look at the code) one option is to simply fork off a new package. Then people can continue to fix bugs and incrementally improve the old version, which would be a huge help for those maintaining existing sites.
But incremental improvements will, before too much longer, come with upgrade scripts.
For now, though, you need to drop your database and reinstall from scratch if you're using PG and want to synch with the latest CVS sources. For this reason I'd recommend you only use it for development sites until we officially declare a "beta" or perhaps even the final release.
Most of the datamodel changes in the PG version were made to enhance performance after initial scalability testing showed that some of our earlier decisions didn't work well in practice, in Postgres.
On the other hand, as has been pointed out the Oracle version's in pretty good shape for rolling out a production site.