I have had that message on both of my last two flights to the US. It meant that I had been SSSSed.

As Israel is quite satisfied with its security procedures, the first time they only told me to go to another counter for an additional security check. There, I was asked to show them both my Israeli and American passports and that was it.

The second time, although the "SSSS" appeared on my boarding pass, Delta's check in agent told me to forget about it -- that they no longer pay any attention to it.

The ironic part was that although Delta wanted me SSSSed in TLV, when I flew back from LAS I was allowed into the TSA Pre-check line -- both times!