multisource can be a pain, and i recommend using trigger_changetargets in place of them where you can, yet i can't think of a way to use them here, so try trigger_relay.
Setup
For your trigger_hurt brush-based entity, use these settings:
name: hurt
damage type: set a damage type, tho i dunno if this is needed
*Also, have the "start off" flag checked on.
Then, you have 2 trigger relays: one called hurt_relay_on and one called hurt_relay_off. have them both target the "hurt" trigger_hurt entity, with obviously the "on" relay's state set to on, and the "off" relay set to off.
Finally, create a multimanager("multithreaded" flag turned on), give it a name, and have your button target it. use settings:
0.01 - hurt_relay_on
4.99 - hurt_relay_off
and if all goes well hitting the button should cause the trigger_hurt to activate for 5 seconds and then shut off. I would also set the button reset time to 5 seconds.