  In gaseous tracking detectors with a large gaseous volume multiple layers of
Gas Electron Multipliers (GEM) can be used to block positive ions from flowing
back into the active volume, which is detrimental to the tracking performance.
We report on studies of effective gain, ion backflow (IBF), and energy
resolution in quadruple GEM detectors, and on strategies for minimizing IBF by
optimizing the operating voltages of the individual GEM layers and the
potential differences between different layers.
