Samba4 does not appear to be in the Hardy (8.04) repositories so there's no easy way (alpha4 is in 8.10, alpha6 in 9.04 and alpha8 in 9.10 & 10.04 repos). With Samba4 still only in alpha build and 10.04 under development now, I doubt we will see an 8.04 backport. I imagine you will be able to try it out relatively easily in the next gen TKL (based on 10.04 rather than 8.04 - prob 6mths +/- away). If you can't wait until then you can try installing from source. It looks like there are instructions on the Samba site, although they don't look like they're for the feint of heart! Have a look here: http://wiki.samba.org/index.php/Samba4/HOWTO

From what I can gather the default version in 10.04 is Samba 3.4.7 (with Samba 4alpha also in the repos) so you should be in luck. The Devs haven't given a timeframe for release but they are currently working on the new one (the most recent release was a maintenance/bugfix release only).

Ubuntu 10.04RC is out now, with the final release coming out end of the month. I would expect the new TKL releases to be available within a few weeks of that.