SMOC answered the ''basics'' here, Jammed Stab.
http://www.pprune.org/rumours-news/5...ml#post8803661
I guess it depends on how deep you wish to go. Delving into Maintainance Manuals and Wiring Schematics might be a little excessive (I was banging my head against a wall on numerous occasions trying to understand them). Anyway, it might be a good idea to find out if your aircraft has the latest and greatest modifications (one being the addition of extra relays and an input from the PSEU to help prevent the premature deactivation of the Altn Ext System by deselecting the Altn Ext Switch/es too early).
As previously stated, the system has to run through a complete cycle. I've read in some manuals, that at least 90 seconds should be allowed before deselecting the Altn Ext switches (presumably on the older type system) to allow this complete cycle.
I was wondering what would happen if there was a sudden (and miraculous) restoration of Hydraulic System 4 with the respective Altn Gear switch still on and the gear lever in UP. Would the gear go up? Would the doors close or stay down? (i.e. does the full alternate cycle reposition the safety valves?) (EDIT: just noticed Mr Peabody's response)