Romulans might even have found out the schedule and timed their attack that way. For all we know, fifty outposts were leveled in this attack, and Outposts 2, 3 and 4 were the only ones that could not be taken down in optimal schedule, so Starfleet learned of their loss and was able to respond - too late to save the lives of thousands of outpost crew, but swiftly enough to avenge.
This scenario seems rather unlikely, according to Commander Hansen on Outpost 4: "Outposts two, three, and eight are gone. Unknown weapon. Completely destroyed, even though we were alerted. Had our deflector shield on maximum. Hit by enormous power. First attack blew our deflector shield. If they hit us again with our deflector shield gone."
For some unknown reason Hansen knows things the Enterprise
doesn't, because he also reports the destruction of Outpost 8, something no one on the Enterprise seems to be aware of and one that is not on the star chart
. His outpost was obviously in scanning range of Outpost 8, the Enterprise
Had there been additional outposts destroyed, he would have probably reported that.
It's also interesting that all the Enterprise learned was that the previous Outposts 2 and 3 went silent. I think this suggests that the moment their deflector shields went up they automatically went silent / communication was cut-off and all "radio power" was rerouted to the shields. While Hansen couldn't hear what happened, obviously his scanners told him what to get ready for.
To me it looks like the Romulans had difficulties recharging their weapon. In contrast to the other outposts, Hansen must have had time and/or extra energy, after his outpost's deflector shields had been obliterated, to send a distress call to the Enterprise
and even have some extra time to wait for them coming into visual communications range.