The incremental update, via App Store, weighed in at 1.44 GB. There were two restarts, both very slow, but everything seems to be working so far.
I've installed it on my server. 10.11.4 requires a new version of OS X Server, and that creates some weird problems (the App Store tries to download the OS X Server update first, then complains that OS X Server won't work with 10.11.3), but once I got that sorted everything worked. No problems so far.