smart iRule that sends a log/trap when 2 members of a 4 member pool are down?

Assuming your virtural server as a default pool defined (IE is not relying on an iRule to define the pool):

when CLIENT_ACCPETED {
if { active_members [LB::server pool] <= 2 } {
   log local0. "ALERT:  pool '[LB::server pool]' has two or less active members".
   }
} 

It's then up to syslog or your monitoring infrastructure to alert on this log message.

Alternately look at HSL here https://devcentral.f5.com/wiki/iRules.HSL__send.ashx for more details on setting up high speed (remote) logging.