The only way, really, to do that kind of light/dark transition is in Av. Shooting in manual you will, as you noted, have problems with the varying light conditions.
Some will say that shooting anything but manual is almost sacrosanct. They feel that shooting in Av introduces flicker as a result of the varying shutter speeds and that in camera exposure meters aren't accurate enough so you'll get a combination of properly exposed, underexposed and overexposed shots. I've come to the conclusion that's pretty much a pantload. It'll happen rarely but not regularly. I've determined that most of the flicker in timelapse clips; mine anyway, is a result of naturally varying light levels that would appear as slow changes over time but appear as flicker when sped up in timelapse.
If you're really concerned about it, there are deflicker filters for Virtualdub; which is a free (Windows only) video editing software, and those filters do a really nice job of reducing or eliminating flicker. Some other video editing apps. have similar filters built in. I use Sony Vegas and it doesn't so I added Vdub to my workflow.
The other thing you may get is wind shake. Even with the camera locked down on a good tripod and even if you put some ballast on the 'pod, if the wind is strong enough, you can get shake from the wind buffeting the tripod. This is far more bothersome, to me, than flicker. Vdub has a deshake filter that also does a really nice job.
In terms of timing, if you only want a 30 second clip from that 4 hour event then your timing is right. A long interval, like 20 seconds, is going to make your 'action' very choppy. You might consider making a longer end product and using intervals in the 5 to 10 second range. When I shoot HDR timelapse, I typically use a 12 second interval and when shooting single shot timelapse it's typically a 6 second interval. The shorter the interval between shots, the smoother the movement in the final video will be.
There are some good timelapse groups at Vimeo where you get a look at some good clips. In some cases, the creators have included the relevant shooting information.http://vimeo.com/channels/timelapseinhdhttp://vimeo.com/groups/hdrtimelapsehttp://vimeo.com/groups/timelapse
I curate the HDR timelapse group.