Measuring precisely the efficiency of a power converter
The four meter approach is pretty standard, though you should be able to calculate your accuracy based on the specs for the meter and the shunts.  If the meters have an averaging function you can use that, possibly along with low pass filtering to get rid of any high frequency measurement noise.  Measure the input and output voltages right at the terminals of the supply of course to avoid the IR drops in the connections and cables.  Nothing wrong with taking several readings and averaging them manually either.