Air Marshals are in an entirely different group than TSA for me and I can't get too upset about Air Marshals only making 4 arrests.
TSA uses invasive, silly and demonstrably ineffective screening techniques to dis-arm normal citizens and Bad Guys (hopefully).
Air Marshals ride in random flights to interdict hi-jacking attempts and other disruptions.
To me, Air Marshals is more like insurance - We all buy it hoping to never use it but glad its there if something were to happen. I mean, we have each bought/own guns, many carry every day but how many of us will ever need to use one in self defense ? And if we do, which day ?, On which trip to the market will it be ??? No one can tell - so the advice is Be Prepared, Always Carry.
My angst with Air Marshal program is that 4000 Marshals can cover only a miniscule percent of the daily flights.... Likely even with heavy analysis of flight data (Plane size, fuel load,etc), No-Fly lists, Passenger Profiling etc, they won't be on the right flight at the right time. Still, we all have to wrestle with how much insurance to buy.
IMHO - Keep the Air Marshals, Get rid of TSA.