How to upgrade multiple Macs to OS X 10.10.4?
For every OS X 'delta' release  - 10.10.1/2/3/ ...10.10.4 etc - which App Store will download to each Mac there is a simultaneous 'combo' update release which rolls up all the previous deltas into one single downloadable update.
It was for many years considered the 'pro' way to do the update, as it can also correct potential update issues that the delta can't.

You can always find updates related to OS X here.

As the combo update doesn't use the App Store "download, install, delete" structure, you can pass it from machine to machine at your leisure.

Maybe not as slick as using the server - but it's free ;-)

From comments -
The combo update can also be applied to machines which are already running the current version of the OS, unlike the App Store's delta "update" mechanism. This can sometimes correct issues that may linger after doing a series of incremental [delta] updates.
The combo updater is invariably larger than the incremental updates; but it is worth it, just for the peace of mind.