  We report on a systematic study of double-gap and four-gap phenolic resistive
plate chambers (RPCs) for future high-{\eta} RPC triggers in the CMS. In the
present study, we constructed real-sized double-gap and four-gap RPCs with gap
thicknesses of 1.6 and 0.8 mm, respectively, with 2-mm-thick phenolic
high-pressure-laminated (HPL) plates. We examined the prototype RPCs for cosmic
rays and 100 GeV muons provided by the SPS H4 beam line at CERN. We applied
maximum gamma rates of 1.5 kHz cm-2 provided by 137Cs sources at Korea
University and the GIF++ irradiation facility installed at the SPS H4 beam line
to examine the rate capabilities of the prototype RPCs. In contrast to the case
of the four-gap RPCs, we found the relatively high threshold was conducive to
effectively suppressing the rapid increase of strip cluster sizes of muon hits
with high voltage, especially when measuring the narrow-pitch strips. The
gamma-induced currents drawn in the four-gap RPC were about one-fourth of those
drawn in the double-gap RPC. The rate capabilities of both RPC types, proven
through the present testing using gamma-ray sources, far exceeded the maximum
rate expected in the new high-{\eta} endcap RPCs planned for future phase-II
LHC runs.
